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btrfs balance fails but though there is lots of free space
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DawgG
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PostPosted: Fri Jul 12, 2019 4:45 pm    Post subject: btrfs balance fails but though there is lots of free space Reply with quote

i recently added a third drive to my btrfs homevol and converted it from raid-1 to raid-5. it consists of three identical 1TB-drives. the fs contains three toplevel subvols (one for /home, two others for ro-snapshots (~150 snaps)
i have lots of free space but btrfs balance fails at about 1% left - this already happened when the conversion from raid-1 to raid-5.

the status now:
Code:
gen2 ~ # btrfs fi sh /home/
Label: 'ROOTVOL-RAID1'  uuid: eacc3df6-aa94-4e87-9acb-878042db68fa
        Total devices 3 FS bytes used 455.24GiB
        devid    1 size 931.51GiB used 230.03GiB path /dev/sdc
        devid    2 size 931.51GiB used 230.03GiB path /dev/sdb
        devid    3 size 931.51GiB used 230.03GiB path /dev/sde


Code:
gen2 ~ # btrfs fi df /home/
Data, RAID5: total=452.00GiB, used=451.14GiB
System, RAID5: total=64.00MiB, used=80.00KiB
Metadata, RAID5: total=8.00GiB, used=4.09GiB
GlobalReserve, single: total=512.00MiB, used=0.00B


to me the above looks good and i did all the tricks mentioned in the btrfs-wiki-page (eg clear_cache, dusage=0, mconvert=raid5,soft, ...) but balance still fails witrh the following error:
last balance (--full-balance)
Code:
[12020.265546] BTRFS info (device sdc): 2 enospc errors during balance
[12020.265548] BTRFS info (device sdc): balance: ended with status: -28
gen2 ~ #  balance: ended with status: -28


what's wrong with this fs?
THX for your input!
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Zucca
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PostPosted: Fri Jul 12, 2019 5:52 pm    Post subject: Reply with quote

Try first converting data only, then metadata (I'd leave metadata as raid1).

Also note that btrfs-raid5 and 6 still aren't considered stable. The write hole still exists.
See: https://btrfs.wiki.kernel.org/index.php/Status
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DawgG
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PostPosted: Tue Jul 16, 2019 8:29 am    Post subject: Reply with quote

THX for your reply!
Quote:
Also note that btrfs-raid5 and 6 still aren't considered stable. The write hole still exists.

damn, i thought i had checked thoroughly enough...

for now i'll completely convert it back to raid-1 and use the spare disk for backups - seems safer (even though the box is not (really) mission-critical...)
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PostPosted: Tue Jul 16, 2019 2:54 pm    Post subject: Reply with quote

I use btrfs-raid10 on my desktop. To work properly it needs at least four devices.
I haven't measured if it boosts performance compared to btrfs-raid1, but at least it's stable. :)
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msst
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PostPosted: Tue Jul 16, 2019 8:41 pm    Post subject: Reply with quote

I would second RAID10 on btrfs. Stable on a backup server here using dmcrypt-btrfs.

With the ever increasing disk sizes and readback-error problems to reconstruct parity on failure I would not want to use RAID5 any more even if it was stable on btrfs. RAID5 save a bit diskspace, but is slower and less safe. With the current prices of HDs an easy choice for me.
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Anon-E-moose
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PostPosted: Tue Jul 16, 2019 8:49 pm    Post subject: Reply with quote

Zucca wrote:
I use btrfs-raid10 on my desktop. To work properly it needs at least four devices.
I haven't measured if it boosts performance compared to btrfs-raid1, but at least it's stable. :)


Doesn't really boost performance over raid 1 as it's a raid 0 (mirror) of raid 1 (stripes).
So 4 disks is the equivalent of a 2 disk stripe as far as speed. Not as fast as 4 disk stripe, but you have redundancy.
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Zucca
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PostPosted: Thu Jul 18, 2019 10:48 pm    Post subject: Reply with quote

Yes. Afaik btrfs-raid10 "splits" files on to two different disks and keep a copy of another two halves on two another disks.
It's basically always a virtual 4-disk raid10.
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