Re: mount option nodatacow for VMs on SSD?
On 2016-11-29 00:06, Duncan wrote: Niccolò Belli posted on Mon, 28 Nov 2016 12:11:49 +0100 as excerpted: On lunedì 28 novembre 2016 09:20:15 CET, Kai Krakow wrote: You can, however, use chattr to make the subvolume root directory (that one where it is mounted) nodatacow (chattr +C) _before_ placing any files or directories in there. That way, newly created files and directories will inherit the flag. Take note that this flag can only applied to directories and empty (zero-sized) files. Do I keep checksumming for this directory such a way? No. Keeping checksums current on NOCOW files is racy, and compression would be complex as well because rewritten data may compress more or less well than the original so the on-filesystem size could change, so both features are disabled in the presence of NOCOW, regardless of how it is set. Put another way, btrfs assumes COW by default and many of its features depend on COW -- that's why these features don't tend to be implemented on conventional rewrite-in-place filesystems in the first place. Both checksumming and compression are among these COW-dependent features. We really should get this put up somewhere very visible on the wiki. It's pretty blatantly obvious to anyone with a CS degree, but most users don't have a CS degree, and I can't count the number of e-mails I've sent explaining why checksums plus NOCOW are a recipe for disaster. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Not TLS] Re: mount option nodatacow for VMs on SSD?
On 2016-11-29 00:14, Duncan wrote: Graham Cobb posted on Mon, 28 Nov 2016 09:49:33 + as excerpted: On 28/11/16 02:56, Duncan wrote: It should still be worth turning on autodefrag on an existing somewhat fragmented filesystem. It just might take some time to defrag files you do modify, and won't touch those you don't, which in some cases might make it worth defragging those manually. Or simply create new filesystems, mount them with autodefrag, and copy everything over so you're starting fresh, as I do. Could that "copy" be (a series of) send/receive, so that snapshots and reflinks are preserved? Does autodefrag work in that case or does the send/receive somehow override that and end up preserving the original (fragmented) extent structure? Very good question that I don't know the answer to as I've not seen it discussed previously. (I'm not a dev, just a list regular and user of btrfs myself, and my personal use-case involves neither snapshots nor send/receive, so on those topics if I've not seen it covered previously either here or on the wiki, I won't know.) Someone else know? Autodefrag does work in that case, but not because there's any special handling for it. In the case of send/receive, the receiving side is doing nothing that couldn't be done as a normal user (except possibly a few ioctls to set subvolume UUID's), so any data it writes will be subject to all processing done by the FS (so sending from an uncompressed volume to one with compress=X will result in the data being compressed on the receiving end too). -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Not TLS] mount option nodatacow for VMs on SSD?
On martedì 29 novembre 2016 06:14:18 CET, Duncan wrote: Very good question that I don't know the answer to as I've not seen it discussed previously. (I'm not a dev, just a list regular and user of btrfs myself, and my personal use-case involves neither snapshots nor send/receive, so on those topics if I've not seen it covered previously either here or on the wiki, I won't know.) Someone else know? Sounds too good to be real, I somehow feel the answer will be "no" :( Niccolò Belli -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Not TLS] Re: mount option nodatacow for VMs on SSD?
Graham Cobb posted on Mon, 28 Nov 2016 09:49:33 + as excerpted: > On 28/11/16 02:56, Duncan wrote: >> It should still be worth turning on autodefrag on an existing somewhat >> fragmented filesystem. It just might take some time to defrag files >> you do modify, and won't touch those you don't, which in some cases >> might make it worth defragging those manually. Or simply create new >> filesystems, mount them with autodefrag, and copy everything over so >> you're starting fresh, as I do. > > Could that "copy" be (a series of) send/receive, so that snapshots and > reflinks are preserved? Does autodefrag work in that case or does the > send/receive somehow override that and end up preserving the original > (fragmented) extent structure? Very good question that I don't know the answer to as I've not seen it discussed previously. (I'm not a dev, just a list regular and user of btrfs myself, and my personal use-case involves neither snapshots nor send/receive, so on those topics if I've not seen it covered previously either here or on the wiki, I won't know.) Someone else know? -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: mount option nodatacow for VMs on SSD?
Niccolò Belli posted on Mon, 28 Nov 2016 12:11:49 +0100 as excerpted: > On lunedì 28 novembre 2016 09:20:15 CET, Kai Krakow wrote: >> You can, however, use chattr to make the subvolume root directory (that >> one where it is mounted) nodatacow (chattr +C) _before_ placing any >> files or directories in there. That way, newly created files and >> directories will inherit the flag. Take note that this flag can only >> applied to directories and empty (zero-sized) files. > > Do I keep checksumming for this directory such a way? No. Keeping checksums current on NOCOW files is racy, and compression would be complex as well because rewritten data may compress more or less well than the original so the on-filesystem size could change, so both features are disabled in the presence of NOCOW, regardless of how it is set. Put another way, btrfs assumes COW by default and many of its features depend on COW -- that's why these features don't tend to be implemented on conventional rewrite-in-place filesystems in the first place. Both checksumming and compression are among these COW-dependent features. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: mount option nodatacow for VMs on SSD?
On lunedì 28 novembre 2016 09:20:15 CET, Kai Krakow wrote: You can, however, use chattr to make the subvolume root directory (that one where it is mounted) nodatacow (chattr +C) _before_ placing any files or directories in there. That way, newly created files and directories will inherit the flag. Take note that this flag can only applied to directories and empty (zero-sized) files. Do I keep checksumming for this directory such a way? Niccolò Belli -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Not TLS] Re: mount option nodatacow for VMs on SSD?
On 28/11/16 02:56, Duncan wrote: > It should still be worth turning on autodefrag on an existing somewhat > fragmented filesystem. It just might take some time to defrag files you > do modify, and won't touch those you don't, which in some cases might > make it worth defragging those manually. Or simply create new > filesystems, mount them with autodefrag, and copy everything over so > you're starting fresh, as I do. Could that "copy" be (a series of) send/receive, so that snapshots and reflinks are preserved? Does autodefrag work in that case or does the send/receive somehow override that and end up preserving the original (fragmented) extent structure? Graham -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: mount option nodatacow for VMs on SSD?
Am Mon, 28 Nov 2016 01:38:29 +0100 schrieb Ulli Horlacher: > On Sat 2016-11-26 (11:27), Kai Krakow wrote: > > > > I have vmware and virtualbox VMs on btrfs SSD. > > > As a side note: I don't think you can use "nodatacow" just for one > > subvolume while the other subvolumes of the same btrfs are mounted > > different. The wiki is just wrong here. > > > > The list of possible mount options in the wiki explicitly lists > > "nodatacow" as not working per subvolume - just globally for the > > whole fs. > > Thanks for pointing this out! > I have misunderstood this, first. You can, however, use chattr to make the subvolume root directory (that one where it is mounted) nodatacow (chattr +C) _before_ placing any files or directories in there. That way, newly created files and directories will inherit the flag. Take note that this flag can only applied to directories and empty (zero-sized) files. That way, you get the intended benefit and your next question applies a little less because: > Ok, then next question :-) > > What is better (for a single user workstation): using mount option > "autodefrag" or call "btrfs filesystem defragment -r" (-t ?) via > nightly cronjob? > > So far, I use neither. When using the above method to make your VM images nodatacow, the only fragmentation issue you need to handle is when doing snapshots. Snapshots are subject to copy-on-write. If you do heavy snapshotting, you'll be getting heavy fragmentation based on the write-patterns. I don't know if autodefrag will handle nodatacow files. You may want to use a dedupe utility after defragmentation, like duperemove (running it manually) or bees (a background daemon also trying to keep fragmentation low). If you are doing no or infrequent snapshots, I won't bother with manual defragging at all for your VM images since you're on SSD. If you aren't going to use snapshots at all, you even won't have to think about autodefrag, tho I still recommend to enable it (see post from Duncan). Manual defrag is a highly write-intensive operation, rewriting multiple gigabytes of data. I strongly recommend against using it on a daily basis for SSD. -- Regards, Kai Replies to list-only preferred. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: mount option nodatacow for VMs on SSD?
Ulli Horlacher posted on Mon, 28 Nov 2016 01:38:29 +0100 as excerpted: > Ok, then next question :-) > > What is better (for a single user workstation): using mount option > "autodefrag" or call "btrfs filesystem defragment -r" (-t ?) via nightly > cronjob? > > So far, I use neither. First point: Be aware that there's a caveat with either method and snapshots, tho it's far stronger with manual defrag than with autodefrag: At one point manual defrag was made snapshot aware, taking care not to deduplicate snapshots and reflinks pointing at the same extents, but the performance penalty of all the extra tracking and calculations turned out to be far too high to be practical with btrfs code in its then-current form (if a defrag run is going to take months, people simply aren't going to run it no matter the claimed benefit), so snapshot/reflink awareness was disabled and it remains so today. AFAIK the plan is still to reenable it, or perhaps make it optional, at some point, but I believe that point remains some distance (years) in the future. Which means for practical purposes, defragging of either type effectively undoes any reflink-based deduplication that may have been done, including that of snapshots -- defrag in the presence of snapshots can double your data space usage. The reason the effect isn't as bad for autodefrag is that while manual defrag can effectively unreflink the extents for entire files regardless of write status, autodefrag only happens in the context of normal file writes or rewrites/modification, and for rewrites/modification, which would COW the modified/rewritten blocks elsewhere in any case, it simply rewrites/relocates rather larger extents, several MiB at a time instead of 4 KiB at a time, than would be the case without autodefrag. So several GiB files that have been snapshotted/reflinked and then modified would have the modified blocks rewritten elsewhere anyway, and autodefrag simply ensures that a large enough new extent (MiB not KiB) is created and rewritten when a single block within it is modified anyway, to avoid the worst fragmentation. It does NOT rewrite and unreflink the entire multi-gig file every time a single block gets modified and written back to the filesystem, as manual defrag can do and in practice often does if there have been modifications since the last snapshot or reflink copy/ dedup of the same file. (Thanks to Hugo for making the point, then checking the actual code and then explaining how autodefrag differs from manual defrag on this point.) So manual recursive defrag of the entire filesystem (as opposed to specific files) is definitely not recommended in btrfs snapshot context, unless you know you have enough space for the snapshot-reflink dedup that the defrag is likely to trigger. But autodefrag should be far more space-conserving in the btrfs snapshotting context, as it'll be far more conservative in what it unreflinks size-wise, and will only unreflink at all when a COW-based modification/rewrite is happening in the first place. Files that remain unchanged will remain safely reflinked to the same extents as those the snapshots hold reflinks to. OTOH, if you're starting out with a highly fragmented existing filesystem, autodefrag can take some time to work its effects, because it *is* far more conservative in what it rewrites and thus defrags. Autodefrag really works best if you handle it as I do here, creating the new filesystem and setting up the mount options to always mount it with autodefrag, before there's any content at all on the filesystem. That way, all files are originally written with autodefrag on, and the filesystem never has a chance to get seriously fragmented in the first place. =:^) It should still be worth turning on autodefrag on an existing somewhat fragmented filesystem. It just might take some time to defrag files you do modify, and won't touch those you don't, which in some cases might make it worth defragging those manually. Or simply create new filesystems, mount them with autodefrag, and copy everything over so you're starting fresh, as I do. (It should be mentioned that in the context of a single write thread on a clean filesystem with lots of free space, a newly written file should always be written in ideal sequential unfragmented form. However, get multiple write threads copying different files at the same time, and even on a new filesystem, the individual files can be fragmented as the various writes intermingle. We've had reports on this list of even brand new distro installations being highly fragmented, and this would appear to be why -- apparently the installer was writing multiple files at once as well as possibly modifying some of them after the initial write, thereby fragmenting them rather heavily. If the installer either mounts with autodefrag before starting to write its files, or if the user either manually creates the
Re: mount option nodatacow for VMs on SSD?
On Sat 2016-11-26 (11:27), Kai Krakow wrote: > > I have vmware and virtualbox VMs on btrfs SSD. > As a side note: I don't think you can use "nodatacow" just for one > subvolume while the other subvolumes of the same btrfs are mounted > different. The wiki is just wrong here. > > The list of possible mount options in the wiki explicitly lists > "nodatacow" as not working per subvolume - just globally for the whole > fs. Thanks for pointing this out! I have misunderstood this, first. Ok, then next question :-) What is better (for a single user workstation): using mount option "autodefrag" or call "btrfs filesystem defragment -r" (-t ?) via nightly cronjob? So far, I use neither. -- Ullrich Horlacher Server und Virtualisierung Rechenzentrum TIK Universitaet Stuttgart E-Mail: horlac...@tik.uni-stuttgart.de Allmandring 30aTel:++49-711-68565868 70569 Stuttgart (Germany) WWW:http://www.tik.uni-stuttgart.de/ REF:<20161126112710.6aca8...@jupiter.sol.kaishome.de> -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: mount option nodatacow for VMs on SSD?
Am Fri, 25 Nov 2016 09:28:40 +0100 schrieb Ulli Horlacher: > I have vmware and virtualbox VMs on btrfs SSD. > > I read in > https://btrfs.wiki.kernel.org/index.php/SysadminGuide#When_To_Make_Subvolumes > > certain types of data (databases, VM images and similar > typically big files that are randomly written internally) may require > CoW to be disabled for them. So for example such areas could be > placed in a subvolume, that is always mounted with the option > "nodatacow". > > Does this apply to SSDs, too? As a side note: I don't think you can use "nodatacow" just for one subvolume while the other subvolumes of the same btrfs are mounted different. The wiki is just wrong here. The list of possible mount options in the wiki explicitly lists "nodatacow" as not working per subvolume - just globally for the whole fs. -- Regards, Kai Replies to list-only preferred. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: mount option nodatacow for VMs on SSD?
On Fri, 25 Nov 2016 12:01:37 + (UTC) Duncan <1i5t5.dun...@cox.net> wrote: > Obviously this can be a HUGE problem on spinning rust due to its seek times, > a problem zero-seek-time ssds don't have They are not strictly zero seek time either. Sure you don't have the issue of moving the physical head around, but still, sequential reads are way faster even on SSDs, compared to random reads. Somewhat typical result for a consumer SSD: Sequential Read : 382.301 MB/s Sequential Write : 315.124 MB/s Random Read 512KB : 261.751 MB/s Random Write 512KB : 334.615 MB/s Random Read 4KB (QD=1) :19.859 MB/s [ 4848.5 IOPS] Random Write 4KB (QD=1) :61.794 MB/s [ 15086.3 IOPS] Random Read 4KB (QD=32) : 132.415 MB/s [ 32327.9 IOPS] Random Write 4KB (QD=32) : 203.051 MB/s [ 49573.0 IOPS] If you have tons of 4K fragments, reading them in can go as low as 20 MB/sec, compared to 382 MB/sec if they were all in one piece. -- With respect, Roman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: mount option nodatacow for VMs on SSD?
Ulli Horlacher posted on Fri, 25 Nov 2016 09:28:40 +0100 as excerpted: > I have vmware and virtualbox VMs on btrfs SSD. > > I read in > https://btrfs.wiki.kernel.org/index.php/SysadminGuide #When_To_Make_Subvolumes > > certain types of data (databases, VM images and similar typically > big files that are randomly written internally) may require CoW to > be disabled for them. So for example such areas could be placed in > a subvolume, that is always mounted with the option "nodatacow". > > Does this apply to SSDs, too? It can, because the root issue is the same, the COW-based fragmentation that's always a problem with this sort of frequently randomly partially rewritten file on COW-based filesystems, but the symptoms tend to be much less of a problem on ssd, so it doesn't tend to be as big of an issue there. On multi-gig database files or VM images, files can end up with 100K extents due to COW-based rewriting. Obviously this can be a HUGE problem on spinning rust due to its seek times, a problem zero-seek-time ssds don't have, but the sheer amount of metadata overhead due to tracking all those tiny extents can be a problem of its own, particularly when doing maintenance such as btrfs balance or btrfs check. Both snapshotting and quota tracking amplify this overhead tracking problem as well, and it's this problem that can still be an issue on ssds. That said, the autodefrag mount option, used to eliminate some of the heavy fragmentation due to copy-on-write (COW) that's the root problem, tends to be faster on ssd, and can often be all that's needed on ssd as between it ameliorating the root problem to a large extent and the faster speed of ssds, often that's all that's needed, particularly if you don't need quotas so have them off and only do relatively limited snapshotting. The problem with both the nodatacow mount option and the nocow file attribute is that they disable some of the btrfs features and are weakened by other features that may well be a big part of the reason behind your choice of btrfs in the first place. Both btrfs compression, if otherwise enabled, and checksuming and thus file integrity checking (and repair in the case of btrfs raid1/10), would be complicated or impossible to implement without COW, and thus are disabled in the NOCOW case. Similarly, btrfs snapshotting depends on COW because the snapshot locks in place the existing version so a rewrite must be written elsewhere. As a result, snapshotting weakens NOCOW to what has been called COW1, COW the first time a block is rewritten after a snapshot, but after that further writes to the same block will be rewritten into the (new) existing block location. If you only do very occasional snapshots that may not be a problem, but if you're doing regular snapshots, particularly automated and multiple per day, the effect of the snapshotting forced COW1s may be fragmentation as bad as if NOCOW wasn't in place in the first place. So to some degree, if you're going to be setting the nocow attribute or using the nodatacow mount option, you might as well just setup a different partition/volume and mkfs to something other than btrfs for those files. OTOH, the btrfs multi-device and storage pool features aren't affected, so if they are big reasons you're doing btrfs, then there's some reason to keep using btrfs and simply do the nodatacow mount or nocow attribute if autodefrag isn't enough on its own to handle it. Bottom line, the fragmentation is much less of a problem on ssds, particularly with autodefrag which may well be enough, but as always, it can be installation and task dependent, so if it's going to be a production system, do your own testing and make your own decisions based on the results. =:^) -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
mount option nodatacow for VMs on SSD?
I have vmware and virtualbox VMs on btrfs SSD. I read in https://btrfs.wiki.kernel.org/index.php/SysadminGuide#When_To_Make_Subvolumes certain types of data (databases, VM images and similar typically big files that are randomly written internally) may require CoW to be disabled for them. So for example such areas could be placed in a subvolume, that is always mounted with the option "nodatacow". Does this apply to SSDs, too? -- Ullrich Horlacher Server und Virtualisierung Rechenzentrum TIK Universitaet Stuttgart E-Mail: horlac...@tik.uni-stuttgart.de Allmandring 30aTel:++49-711-68565868 70569 Stuttgart (Germany) WWW:http://www.tik.uni-stuttgart.de/ REF:<20161125082840.ga32...@rus.uni-stuttgart.de> -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html