Hello group. I am confused: Can somebody please confirm/deny, which RAID subsystem is affected? BTRFS' RAID5/6 or mdadm (Linux kernel raid) RAID 5/6 ?
Are there some gotchas (in terms of broken reliability) when using kernel one? The web is full of legends, it seems that this confusion is quite common... On 06/21/2017 12:57 AM, waxhead wrote: > I am trying to piece together the actual status of the RAID5/6 bit of > BTRFS. > The wiki refer to kernel 3.19 which was released in February 2015 so I > assume that the information there is a tad outdated (the last update > on the wiki page was July 2016) > https://btrfs.wiki.kernel.org/index.php/RAID56 > > Now there are four problems listed > > 1. Parity may be inconsistent after a crash (the "write hole") > Is this still true, if yes - would not this apply for RAID1 / RAID10 > as well? How was it solved there , and why can't that be done for RAID5/6 > > 2. Parity data is not checksummed > Why is this a problem? Does it have to do with the design of BTRFS > somehow? > Parity is after all just data, BTRFS does checksum data so what is the > reason this is a problem? > > 3. No support for discard? (possibly -- needs confirmation with cmason) > Does this matter that much really?, is there an update on this? > > 4. The algorithm uses as many devices as are available: No support for > a fixed-width stripe. > What is the plan for this one? There was patches on the mailing list > by the SnapRAID author to support up to 6 parity devices. Will the > (re?) resign of btrfs raid5/6 support a scheme that allows for > multiple parity devices? > > I do have a few other questions as well... > > 5. BTRFS does still (kernel 4.9) not seem to use the device ID to > communicate with devices. > > If you on a multi device filesystem yank out a device, for example > /dev/sdg and it reappear as /dev/sdx for example btrfs will still > happily try to write to /dev/sdg even if btrfs fi sh /mnt shows the > correct device ID. What is the status for getting BTRFS to properly > understand that a device is missing? > > 6. RAID1 needs to be able to make two copies always. E.g. if you have > three disks you can loose one and it should still work. What about > RAID10 ? If you have for example 6 disk RAID10 array, loose one disk > and reboots (due to #5 above). Will RAID10 recognize that the array > now is a 5 disk array and stripe+mirror over 2 disks (or possibly 2.5 > disks?) instead of 3? In other words, will it work as long as it can > create a RAID10 profile that requires a minimum of four disks?
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