Re: [linux-lvm] LVM performance vs direct dm-thin
Il 2022-01-30 22:17 Demi Marie Obenour ha scritto: On Xen, the paravirtualised block backend driver (blkback) requires a block device, so file-based virtual disks are implemented with a loop device managed by the toolstack. Suggestions for improving this less-than-satisfactory situation are welcome. Ah - I expected that with something as disk = [ 'file:mydisk.img,hda,w' ] Xen would have directly used "mydisk.img" as the backend disk file. Does it instead automatically create a loopback overlay? I mainly use KVM, and maybe I am spoiled by its capability to use basically any datastore as backing disk. Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.da...@assyoma.it - i...@assyoma.it GPG public key ID: FF5F32A8 ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
Il 2022-01-30 22:39 Stuart D. Gathman ha scritto: I use LVM as flexible partitions (i.e. only classic LVs, no thin pool). Classic LVs perform like partitions, literally using the same driver (device mapper) with a small number of extents, and are if anything more recoverable than partition tables. We used to put LVM on bare drives (like AIX did) - who needs a partition table? But on Wintel, you need a partition table for EFI and so that alien operating systems know there is something already on a disk. Classical (fat) LVs are rock solid, but how do you cope with fast (maybe rolling) snapshotting? This is the main selling point of thinlvm. Since we use LVs like partitions - mixing with btrfs is not an issue. Just use the LVs like partitions. I haven't tried ZFS on linux - it may have LVM like features that could fight with LVM. ZFS would be my first choice on a BSD box. I broadly use ZFS - and yes, it is a wonderful tools. Than said, it has its own gotcha. For example: - snapshot rollback is a destructive operation (ie: after rollback, you permanently lose the current filesystem state); - clones (writable snapshots) depend on the read-only base image (ie: on the original snapshot), which you can not delete until you have its clones around. Moreover, snapshotting/cloning a ZFS dataset (or volume) does not appear to be significantly faster then LVM - sometime it requires ~1s, depending on the load. We do not use LVM raid - but either run mdraid underneath, or let btrfs do it's data duplication thing with LVs on different spindles. I always found btrfs very underperforming when facing random rewrite workloads as VMs and DBs. Can I ask your experience? Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.da...@assyoma.it - i...@assyoma.it GPG public key ID: FF5F32A8 ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
On Sun, Jan 30, 2022 at 04:39:30PM -0500, Stuart D. Gathman wrote: > On Sun, 2022-01-30 at 11:45 -0500, Demi Marie Obenour wrote: > > On Sun, Jan 30, 2022 at 11:52:52AM +0100, Zdenek Kabelac wrote: > > > > > > > > Since you mentioned ZFS - you might want focus on using 'ZFS-only' > > > solution. > > > Combining ZFS or Btrfs with lvm2 is always going to be a painful > > > way as > > > those filesystems have their own volume management. > > > > Absolutely! That said, I do wonder what your thoughts on using loop > > devices for VM storage are. I know they are slower than thin > > volumes, > > but they are also much easier to manage, since they are just ordinary > > disk files. Any filesystem with reflink can provide the needed > > copy-on-write support. > > I use loop devices for test cases - especially with simulated IO > errors. Devs really appreciate having an easy reproducer for > database/filesystem bugs (which often involve handling of IO errors). > But not for production VMs. > > I use LVM as flexible partitions (i.e. only classic LVs, no thin pool). > Classic LVs perform like partitions, literally using the same driver > (device mapper) with a small number of extents, and are if anything > more recoverable than partition tables. We used to put LVM on bare > drives (like AIX did) - who needs a partition table? But on Wintel, > you need a partition table for EFI and so that alien operating systems > know there is something already on a disk. > > Your VM usage is different from ours - you seem to need to clone and > activate a VM quickly (like a vps provider might need to do). We > generally have to buy more RAM to add a new VM :-), so performance of > creating a new LV is the least of our worries. To put it mildly, yes :). Ideally we could get VM boot time down to 100ms or lower. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab signature.asc Description: PGP signature ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
On Sun, 2022-01-30 at 11:45 -0500, Demi Marie Obenour wrote: > On Sun, Jan 30, 2022 at 11:52:52AM +0100, Zdenek Kabelac wrote: > > > > > Since you mentioned ZFS - you might want focus on using 'ZFS-only' > > solution. > > Combining ZFS or Btrfs with lvm2 is always going to be a painful > > way as > > those filesystems have their own volume management. > > Absolutely! That said, I do wonder what your thoughts on using loop > devices for VM storage are. I know they are slower than thin > volumes, > but they are also much easier to manage, since they are just ordinary > disk files. Any filesystem with reflink can provide the needed > copy-on-write support. I use loop devices for test cases - especially with simulated IO errors. Devs really appreciate having an easy reproducer for database/filesystem bugs (which often involve handling of IO errors). But not for production VMs. I use LVM as flexible partitions (i.e. only classic LVs, no thin pool). Classic LVs perform like partitions, literally using the same driver (device mapper) with a small number of extents, and are if anything more recoverable than partition tables. We used to put LVM on bare drives (like AIX did) - who needs a partition table? But on Wintel, you need a partition table for EFI and so that alien operating systems know there is something already on a disk. Your VM usage is different from ours - you seem to need to clone and activate a VM quickly (like a vps provider might need to do). We generally have to buy more RAM to add a new VM :-), so performance of creating a new LV is the least of our worries. Since we use LVs like partitions - mixing with btrfs is not an issue. Just use the LVs like partitions. I haven't tried ZFS on linux - it may have LVM like features that could fight with LVM. ZFS would be my first choice on a BSD box. We do not use LVM raid - but either run mdraid underneath, or let btrfs do it's data duplication thing with LVs on different spindles. ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
On Sun, Jan 30, 2022 at 09:27:56PM +0100, Gionatan Danti wrote: > Il 2022-01-30 18:43 Zdenek Kabelac ha scritto: > > Chain filesystem->block_layer->filesystem->block_layer is something > > you most likely do not want to use for any well performing solution... > > But it's ok for testing... > > I second that. > > Demi Marie - just a question: are you sure do you really needs a block > device? I don't know QubeOS, but both KVM and Xen can use files as virtual > disks. This would enable you to ignore loopback mounts. On Xen, the paravirtualised block backend driver (blkback) requires a block device, so file-based virtual disks are implemented with a loop device managed by the toolstack. Suggestions for improving this less-than-satisfactory situation are welcome. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab signature.asc Description: PGP signature ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] Running thin_trim before activating a thin pool
Il 2022-01-30 12:18 Zdenek Kabelac ha scritto: > Thin is more oriented towards extreme speed. VDO is more about 'compression & deduplication' - so space efficiency. Combining both together is kind of harming their advantages. Unfortunately, it is the only (current) solution to have snapshotting with data compression/deduplication. Integrating snapshot capability into VDO would be awesome! Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.da...@assyoma.it - i...@assyoma.it GPG public key ID: FF5F32A8 ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
Il 2022-01-30 18:43 Zdenek Kabelac ha scritto: Chain filesystem->block_layer->filesystem->block_layer is something you most likely do not want to use for any well performing solution... But it's ok for testing... I second that. Demi Marie - just a question: are you sure do you really needs a block device? I don't know QubeOS, but both KVM and Xen can use files as virtual disks. This would enable you to ignore loopback mounts. Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.da...@assyoma.it - i...@assyoma.it GPG public key ID: FF5F32A8 ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] Running thin_trim before activating a thin pool
Dne 30. 01. 22 v 19:01 Demi Marie Obenour napsal(a): On Sun, Jan 30, 2022 at 06:56:43PM +0100, Zdenek Kabelac wrote: Dne 30. 01. 22 v 18:30 Demi Marie Obenour napsal(a): On Sun, Jan 30, 2022 at 12:18:32PM +0100, Zdenek Kabelac wrote: Then are always landing in upstream kernel once they are all validated & tested (recent kernel already has many speed enhancements). Thanks! Which mailing list should I be watching? lkml You could easily run in parallel individual blkdiscards for your thin LVs For most modern drives thought it's somewhat waste of time... Those trimming tools should be used when they are solving some real problems, running them just for fun is just energy & performance waste My understanding (which could be wrong) is that periodic trim is necessary for SSDs. This was useful for archaic SSDs. Modern SSD/NVMe drives are much smarter... Regards Zdenek ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] Running thin_trim before activating a thin pool
On Sun, Jan 30, 2022 at 06:56:43PM +0100, Zdenek Kabelac wrote: > Dne 30. 01. 22 v 18:30 Demi Marie Obenour napsal(a): > > On Sun, Jan 30, 2022 at 12:18:32PM +0100, Zdenek Kabelac wrote: > > > Discard of thins itself is AFAIC pretty fast - unless you have massively > > > sized thin devices with many GiB of metadata - obviously you cannot > > > process > > > this amount of metadata in nanoseconds (and there are prepared kernel > > > patches to make it even faster) > > > > Would you be willing and able to share those patches? > > Then are always landing in upstream kernel once they are all validated & > tested (recent kernel already has many speed enhancements). Thanks! Which mailing list should I be watching? > > > What is the problem is the speed of discard of physical devices. > > > You could actually try to feel difference with: > > > lvchange --discards passdown|nopassdown thinpool > > > > In Qubes OS I believe we do need the discards to be passed down > > eventually, but I doubt it needs to be synchronous. Being able to run > > the equivalent of `fstrim -av` periodically would be amazing. I’m > > CC’ing Marek Marczykowski-Górecki (Qubes OS project lead) in case he > > has something to say. > > You could easily run in parallel individual blkdiscards for your thin LVs > For most modern drives thought it's somewhat waste of time... > > Those trimming tools should be used when they are solving some real > problems, running them just for fun is just energy & performance waste My understanding (which could be wrong) is that periodic trim is necessary for SSDs. > > > Also it's very important to keep metadata on fast storage device > > > (SSD/NVMe)! > > > Keeping metadata on same hdd spindle as data is always going to feel slow > > > (in fact it's quite pointless to talk about performance and use hdd...) > > > > That explains why I had such a horrible experience with my initial > > (split between NVMe and HDD) install. I would not be surprised if some > > or all of the metadata volume wound up on the spinning disk. > > With lvm2 user can always 'pvmove' any LV to any desired PV. > There is not yet any 'smart' logic to do it automatically. Good point. I was probably unware of that at the time. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab signature.asc Description: PGP signature ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] Running thin_trim before activating a thin pool
Dne 30. 01. 22 v 18:30 Demi Marie Obenour napsal(a): On Sun, Jan 30, 2022 at 12:18:32PM +0100, Zdenek Kabelac wrote: Dne 30. 01. 22 v 2:20 Demi Marie Obenour napsal(a): On Sat, Jan 29, 2022 at 10:40:34PM +0100, Zdenek Kabelac wrote: Dne 29. 01. 22 v 21:09 Demi Marie Obenour napsal(a): On Sat, Jan 29, 2022 at 08:42:21PM +0100, Zdenek Kabelac wrote: Dne 29. 01. 22 v 19:52 Demi Marie Obenour napsal(a): Discard of thins itself is AFAIC pretty fast - unless you have massively sized thin devices with many GiB of metadata - obviously you cannot process this amount of metadata in nanoseconds (and there are prepared kernel patches to make it even faster) Would you be willing and able to share those patches? Then are always landing in upstream kernel once they are all validated & tested (recent kernel already has many speed enhancements). What is the problem is the speed of discard of physical devices. You could actually try to feel difference with: lvchange --discards passdown|nopassdown thinpool In Qubes OS I believe we do need the discards to be passed down eventually, but I doubt it needs to be synchronous. Being able to run the equivalent of `fstrim -av` periodically would be amazing. I’m CC’ing Marek Marczykowski-Górecki (Qubes OS project lead) in case he has something to say. You could easily run in parallel individual blkdiscards for your thin LVs For most modern drives thought it's somewhat waste of time... Those trimming tools should be used when they are solving some real problems, running them just for fun is just energy & performance waste Also it's very important to keep metadata on fast storage device (SSD/NVMe)! Keeping metadata on same hdd spindle as data is always going to feel slow (in fact it's quite pointless to talk about performance and use hdd...) That explains why I had such a horrible experience with my initial (split between NVMe and HDD) install. I would not be surprised if some or all of the metadata volume wound up on the spinning disk. With lvm2 user can always 'pvmove' any LV to any desired PV. There is not yet any 'smart' logic to do it automatically. add support for efficient snapshots of data stored on a VDO volume, and to have multiple volumes on top of a single VDO volume. Furthermore, We hope we will add some direct 'snapshot' support to VDO so users will not need to combine both technologies together. Does that include support for splitting a VDO volume into multiple, individually-snapshottable volumes, the way thin works? Yes - that's the plan - to have multiple VDO LV in a single VDOPool. Regards Zdenek ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
Dne 30. 01. 22 v 17:45 Demi Marie Obenour napsal(a): On Sun, Jan 30, 2022 at 11:52:52AM +0100, Zdenek Kabelac wrote: Dne 30. 01. 22 v 1:32 Demi Marie Obenour napsal(a): On Sat, Jan 29, 2022 at 10:32:52PM +0100, Zdenek Kabelac wrote: Dne 29. 01. 22 v 21:34 Demi Marie Obenour napsal(a): How much slower are operations on an LVM2 thin pool compared to manually managing a dm-thin target via ioctls? I am mostly concerned about volume snapshot, creation, and destruction. Data integrity is very important, so taking shortcuts that risk data loss is out of the question. However, the application may have some additional information that LVM2 does not have. For instance, it may know that the volume that it is snapshotting is not in use, or that a certain volume it is creating will never be used after power-off. So brave developers may always write their own management tools for their constrained environment requirements that will by significantly faster in terms of how many thins you could create per minute (btw you will need to also consider dropping usage of udev on such system) What kind of constraints are you referring to? Is it possible and safe to have udev running, but told to ignore the thins in question? Lvm2 is oriented more towards managing set of different disks, where user is adding/removing/replacing them. So it's more about recoverability, good support for manual repair (ascii metadata), tracking history of changes, backward compatibility, support of conversion to different volume types (i.e. caching of thins, pvmove...) Support for no/udev & no/systemd, clusters and nearly every linux distro available... So there is a lot - and this all adds quite complexity. I am certain it does, and that makes a lot of sense. Thanks for the hard work! Those features are all useful for Qubes OS, too — just not in the VM startup/shutdown path. So once you scratch all this - and you say you only care about single disc then you are able to use more efficient metadata formats which you could even keep permanently in memory during the lifetime - this all adds great performance. But it all depends how you could constrain your environment. It's worth to mention there is lvm2 support for 'external' 'thin volume' creators - so lvm2 only maintains 'thin-pool' data & metadata LV - but thin volume creation, activation, deactivation of thins is left to external tool. This has been used by docker for a while - later on they switched to overlayFs I believe.. That indeeds sounds like a good choice for Qubes OS. It would allow the data and metadata LVs to be any volume type that lvm2 supports, and managed using all of lvm2’s features. So one could still put the metadata on a RAID-10 volume while everything else is RAID-6, or set up a dm-cache volume to store the data (please correct me if I am wrong). Qubes OS has already moved to using a separate thin pool for virtual machines, as it prevents dom0 (privileged management VM) from being run out of disk space (by accident or malice). That means that the thin pool use for guests is managed only by Qubes OS, and so the standard lvm2 tools do not need to touch it. Is this a setup that you would recommend, and would be comfortable using in production? As far as metadata is concerned, Qubes OS has its own XML file containing metadata about all qubes, which should suffice for this purpose. To prevent races during updates and ensure automatic crash recovery, is it sufficient to store metadata for both new and old transaction IDs, and pick the correct one based on the device-mapper status line? I have seen lvm2 get in an inconsistent state (transaction ID off by one) that required manual repair before, which is quite unnerving for a desktop OS. My biased advice would be to stay with lvm2. There is lot of work, many things are not well documented and getting everything running correctly will take a lot of effort (Docker in fact did not managed to do it well and was incapable to provide any recoverability) One feature that would be nice is to be able to import an externally-provided mapping of thin pool device numbers to LV names, so that lvm2 could provide a (read-only, and not guaranteed fresh) view of system state for reporting purposes. Once you will have evidence it's the lvm2 causing major issue - you could consider whether it's worth to step into a separate project. It's worth to mention - the more bullet-proof you will want to make your project - the more closer to the extra processing made by lvm2 you will get. Why is this? How does lvm2 compare to stratis, for example? Stratis is yet another volume manager written in Rust combined with XFS for easier user experience. That's all I'd probably say about it... That’s fine. I guess my question is why making lvm2 bullet-proof needs so much overhead. It's difficult - if you would be distributing lvm2 with exact kernel version & udev & systemd with a single linux distro - it
Re: [linux-lvm] Running thin_trim before activating a thin pool
On Sun, Jan 30, 2022 at 12:18:32PM +0100, Zdenek Kabelac wrote: > Dne 30. 01. 22 v 2:20 Demi Marie Obenour napsal(a): > > On Sat, Jan 29, 2022 at 10:40:34PM +0100, Zdenek Kabelac wrote: > > > Dne 29. 01. 22 v 21:09 Demi Marie Obenour napsal(a): > > > > On Sat, Jan 29, 2022 at 08:42:21PM +0100, Zdenek Kabelac wrote: > > > > > Dne 29. 01. 22 v 19:52 Demi Marie Obenour napsal(a): > > > > > > Is it possible to configure LVM2 so that it runs thin_trim before it > > > > > > activates a thin pool? Qubes OS currently runs blkdiscard on every > > > > > > thin > > > > > > volume before deleting it, which is slow and unreliable. Would > > > > > > running > > > > > > thin_trim during system startup provide a better alternative? > > > > > > > > > > Hi > > > > > > > > > > > > > > > Nope there is currently no support from lvm2 side for this. > > > > > Feel free to open RFE. > > > > > > > > Done: https://bugzilla.redhat.com/show_bug.cgi?id=2048160 > > > > > > > > > > > > > > Thanks > > > > > > Although your use-case Thinpool on top of VDO is not really a good plan > > > and > > > there is a good reason behind why lvm2 does not support this device stack > > > directly (aka thin-pool data LV as VDO LV). > > > I'd say you are stepping on very very thin ice... > > > > Thin pool on VDO is not my actual use-case. The actual reason for the > > ticket is slow discards of thin devices that are about to be deleted; > > Hi > > Discard of thins itself is AFAIC pretty fast - unless you have massively > sized thin devices with many GiB of metadata - obviously you cannot process > this amount of metadata in nanoseconds (and there are prepared kernel > patches to make it even faster) Would you be willing and able to share those patches? > What is the problem is the speed of discard of physical devices. > You could actually try to feel difference with: > lvchange --discards passdown|nopassdown thinpool In Qubes OS I believe we do need the discards to be passed down eventually, but I doubt it needs to be synchronous. Being able to run the equivalent of `fstrim -av` periodically would be amazing. I’m CC’ing Marek Marczykowski-Górecki (Qubes OS project lead) in case he has something to say. > Also it's very important to keep metadata on fast storage device (SSD/NVMe)! > Keeping metadata on same hdd spindle as data is always going to feel slow > (in fact it's quite pointless to talk about performance and use hdd...) That explains why I had such a horrible experience with my initial (split between NVMe and HDD) install. I would not be surprised if some or all of the metadata volume wound up on the spinning disk. > > you can find more details in the linked GitHub issue. That said, now I > > am curious why you state that dm-thin on top of dm-vdo (that is, > > userspace/filesystem/VM/etc ⇒ dm-thin data (*not* metadata) ⇒ dm-vdo ⇒ > > hardware/dm-crypt/etc) is a bad idea. It seems to be a decent way to > > Out-of-space recoveries are ATM much harder then what we want. Okay, thanks! Will this be fixed in a future version? > So as long as user can maintain free space of your VDO and thin-pool it's > ok. Once user runs out of space - recovery is pretty hard task (and there is > reason we have support...) Out of space is already a tricky issue in Qubes OS. I certainly would not want to make it worse. > > add support for efficient snapshots of data stored on a VDO volume, and > > to have multiple volumes on top of a single VDO volume. Furthermore, > > We hope we will add some direct 'snapshot' support to VDO so users will not > need to combine both technologies together. Does that include support for splitting a VDO volume into multiple, individually-snapshottable volumes, the way thin works? > Thin is more oriented towards extreme speed. > VDO is more about 'compression & deduplication' - so space efficiency. > > Combining both together is kind of harming their advantages. That makes sense. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab signature.asc Description: PGP signature ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
On Sun, Jan 30, 2022 at 11:52:52AM +0100, Zdenek Kabelac wrote: > Dne 30. 01. 22 v 1:32 Demi Marie Obenour napsal(a): > > On Sat, Jan 29, 2022 at 10:32:52PM +0100, Zdenek Kabelac wrote: > > > Dne 29. 01. 22 v 21:34 Demi Marie Obenour napsal(a): > > > > How much slower are operations on an LVM2 thin pool compared to manually > > > > managing a dm-thin target via ioctls? I am mostly concerned about > > > > volume snapshot, creation, and destruction. Data integrity is very > > > > important, so taking shortcuts that risk data loss is out of the > > > > question. However, the application may have some additional information > > > > that LVM2 does not have. For instance, it may know that the volume that > > > > it is snapshotting is not in use, or that a certain volume it is > > > > creating will never be used after power-off. > > > > > > > > > So brave developers may always write their own management tools for their > > > constrained environment requirements that will by significantly faster in > > > terms of how many thins you could create per minute (btw you will need to > > > also consider dropping usage of udev on such system) > > > > What kind of constraints are you referring to? Is it possible and safe > > to have udev running, but told to ignore the thins in question? > > Lvm2 is oriented more towards managing set of different disks, > where user is adding/removing/replacing them. So it's more about > recoverability, good support for manual repair (ascii metadata), > tracking history of changes, backward compatibility, support > of conversion to different volume types (i.e. caching of thins, pvmove...) > Support for no/udev & no/systemd, clusters and nearly every linux distro > available... So there is a lot - and this all adds quite complexity. I am certain it does, and that makes a lot of sense. Thanks for the hard work! Those features are all useful for Qubes OS, too — just not in the VM startup/shutdown path. > So once you scratch all this - and you say you only care about single disc > then you are able to use more efficient metadata formats which you could > even keep permanently in memory during the lifetime - this all adds great > performance. > > But it all depends how you could constrain your environment. > > It's worth to mention there is lvm2 support for 'external' 'thin volume' > creators - so lvm2 only maintains 'thin-pool' data & metadata LV - but thin > volume creation, activation, deactivation of thins is left to external tool. > This has been used by docker for a while - later on they switched to > overlayFs I believe.. That indeeds sounds like a good choice for Qubes OS. It would allow the data and metadata LVs to be any volume type that lvm2 supports, and managed using all of lvm2’s features. So one could still put the metadata on a RAID-10 volume while everything else is RAID-6, or set up a dm-cache volume to store the data (please correct me if I am wrong). Qubes OS has already moved to using a separate thin pool for virtual machines, as it prevents dom0 (privileged management VM) from being run out of disk space (by accident or malice). That means that the thin pool use for guests is managed only by Qubes OS, and so the standard lvm2 tools do not need to touch it. Is this a setup that you would recommend, and would be comfortable using in production? As far as metadata is concerned, Qubes OS has its own XML file containing metadata about all qubes, which should suffice for this purpose. To prevent races during updates and ensure automatic crash recovery, is it sufficient to store metadata for both new and old transaction IDs, and pick the correct one based on the device-mapper status line? I have seen lvm2 get in an inconsistent state (transaction ID off by one) that required manual repair before, which is quite unnerving for a desktop OS. One feature that would be nice is to be able to import an externally-provided mapping of thin pool device numbers to LV names, so that lvm2 could provide a (read-only, and not guaranteed fresh) view of system state for reporting purposes. > > > It's worth to mention - the more bullet-proof you will want to make your > > > project - the more closer to the extra processing made by lvm2 you will > > > get. > > > > Why is this? How does lvm2 compare to stratis, for example? > > Stratis is yet another volume manager written in Rust combined with XFS for > easier user experience. That's all I'd probably say about it... That’s fine. I guess my question is why making lvm2 bullet-proof needs so much overhead. > > > However before you will step into these waters - you should probably > > > evaluate whether thin-pool actually meet your needs if you have that high > > > expectation for number of supported volumes - so you will not end up with > > > hyper fast snapshot creation while the actual usage then is not meeting > > > your > > > needs... > > > > What needs are you thinking of specifically? Qubes OS needs block >
Re: [linux-lvm] Running thin_trim before activating a thin pool
Dne 30. 01. 22 v 2:20 Demi Marie Obenour napsal(a): On Sat, Jan 29, 2022 at 10:40:34PM +0100, Zdenek Kabelac wrote: Dne 29. 01. 22 v 21:09 Demi Marie Obenour napsal(a): On Sat, Jan 29, 2022 at 08:42:21PM +0100, Zdenek Kabelac wrote: Dne 29. 01. 22 v 19:52 Demi Marie Obenour napsal(a): Is it possible to configure LVM2 so that it runs thin_trim before it activates a thin pool? Qubes OS currently runs blkdiscard on every thin volume before deleting it, which is slow and unreliable. Would running thin_trim during system startup provide a better alternative? Hi Nope there is currently no support from lvm2 side for this. Feel free to open RFE. Done: https://bugzilla.redhat.com/show_bug.cgi?id=2048160 Thanks Although your use-case Thinpool on top of VDO is not really a good plan and there is a good reason behind why lvm2 does not support this device stack directly (aka thin-pool data LV as VDO LV). I'd say you are stepping on very very thin ice... Thin pool on VDO is not my actual use-case. The actual reason for the ticket is slow discards of thin devices that are about to be deleted; Hi Discard of thins itself is AFAIC pretty fast - unless you have massively sized thin devices with many GiB of metadata - obviously you cannot process this amount of metadata in nanoseconds (and there are prepared kernel patches to make it even faster) What is the problem is the speed of discard of physical devices. You could actually try to feel difference with: lvchange --discards passdown|nopassdown thinpool Also it's very important to keep metadata on fast storage device (SSD/NVMe)! Keeping metadata on same hdd spindle as data is always going to feel slow (in fact it's quite pointless to talk about performance and use hdd...) you can find more details in the linked GitHub issue. That said, now I am curious why you state that dm-thin on top of dm-vdo (that is, userspace/filesystem/VM/etc ⇒ dm-thin data (*not* metadata) ⇒ dm-vdo ⇒ hardware/dm-crypt/etc) is a bad idea. It seems to be a decent way to Out-of-space recoveries are ATM much harder then what we want. So as long as user can maintain free space of your VDO and thin-pool it's ok. Once user runs out of space - recovery is pretty hard task (and there is reason we have support...) add support for efficient snapshots of data stored on a VDO volume, and to have multiple volumes on top of a single VDO volume. Furthermore, We hope we will add some direct 'snapshot' support to VDO so users will not need to combine both technologies together. Thin is more oriented towards extreme speed. VDO is more about 'compression & deduplication' - so space efficiency. Combining both together is kind of harming their advantages. https://access.redhat.com/articles/2106521#vdo recommends exactly this use-case. Or am I misunderstanding you? There are many paths to Rome... So as mentioned above - you need to pick performance/space effieciency. And since you want to write your own thin volume managing software, I'm guessing you care about performance a lot (so we do - but with our given constrains that are limiting us to some level)... Also I assume you have already checked performance of discard on VDO, but I would not want to run this operation frequently on any larger volume... I have never actually used VDO myself, although the documentation does warn about this. It's been purely related to the initial BZ description which cares a lot about thin discard performance and following comment adds VDO discard into same equation... :) Regards Zdenek ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
Re: [linux-lvm] LVM performance vs direct dm-thin
Dne 30. 01. 22 v 1:32 Demi Marie Obenour napsal(a): On Sat, Jan 29, 2022 at 10:32:52PM +0100, Zdenek Kabelac wrote: Dne 29. 01. 22 v 21:34 Demi Marie Obenour napsal(a): How much slower are operations on an LVM2 thin pool compared to manually managing a dm-thin target via ioctls? I am mostly concerned about volume snapshot, creation, and destruction. Data integrity is very important, so taking shortcuts that risk data loss is out of the question. However, the application may have some additional information that LVM2 does not have. For instance, it may know that the volume that it is snapshotting is not in use, or that a certain volume it is creating will never be used after power-off. So brave developers may always write their own management tools for their constrained environment requirements that will by significantly faster in terms of how many thins you could create per minute (btw you will need to also consider dropping usage of udev on such system) What kind of constraints are you referring to? Is it possible and safe to have udev running, but told to ignore the thins in question? Lvm2 is oriented more towards managing set of different disks, where user is adding/removing/replacing them. So it's more about recoverability, good support for manual repair (ascii metadata), tracking history of changes, backward compatibility, support of conversion to different volume types (i.e. caching of thins, pvmove...) Support for no/udev & no/systemd, clusters and nearly every linux distro available... So there is a lot - and this all adds quite complexity. So once you scratch all this - and you say you only care about single disc then you are able to use more efficient metadata formats which you could even keep permanently in memory during the lifetime - this all adds great performance. But it all depends how you could constrain your environment. It's worth to mention there is lvm2 support for 'external' 'thin volume' creators - so lvm2 only maintains 'thin-pool' data & metadata LV - but thin volume creation, activation, deactivation of thins is left to external tool. This has been used by docker for a while - later on they switched to overlayFs I believe.. It's worth to mention - the more bullet-proof you will want to make your project - the more closer to the extra processing made by lvm2 you will get. Why is this? How does lvm2 compare to stratis, for example? Stratis is yet another volume manager written in Rust combined with XFS for easier user experience. That's all I'd probably say about it... However before you will step into these waters - you should probably evaluate whether thin-pool actually meet your needs if you have that high expectation for number of supported volumes - so you will not end up with hyper fast snapshot creation while the actual usage then is not meeting your needs... What needs are you thinking of specifically? Qubes OS needs block devices, so filesystem-backed storage would require the use of loop devices unless I use ZFS zvols. Do you have any specific recommendations? As long as you live in the world without crashes, buggy kernels, apps and failing hard drives everything looks very simple. And every development costs quite some time & money. Since you mentioned ZFS - you might want focus on using 'ZFS-only' solution. Combining ZFS or Btrfs with lvm2 is always going to be a painful way as those filesystems have their own volume management. Regards Zdenek ___ linux-lvm mailing list linux-lvm@redhat.com https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
