[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM
zkabelac at redhat.com
Wed Apr 26 14:33:15 UTC 2017
Dne 26.4.2017 v 15:37 Gionatan Danti napsal(a):
> On 26/04/2017 13:23, Zdenek Kabelac wrote:
>> You need to use 'direct' write more - otherwise you are just witnessing
>> issues related with 'page-cache' flushing.
>> Every update of file means update of journal - so you surely can lose
>> some data in-flight - but every good software needs to the flush before
>> doing next transaction - so with correctly working transaction software
>> no data could be lost.
> I used "oflag=sync" for this very reason - to avoid async writes, However,
> let's retry with "oflat=direct,sync".
> This is the thinpool before filling:
> [root at blackhole mnt]# lvs
> LV VG Attr LSize Pool Origin Data% Meta% Move Log
> Cpy%Sync Convert
> thinpool vg_kvm twi-aot--- 1.00g 87.66 12.01
> thinvol vg_kvm Vwi-aot--- 2.00g thinpool 43.83
> root vg_system -wi-ao---- 50.00g
> swap vg_system -wi-ao---- 7.62g
> [root at blackhole storage]# mount | grep thinvol
> /dev/mapper/vg_kvm-thinvol on /mnt/storage type ext4
> Fill the thin volume (note that errors are raised immediately due to
> [root at blackhole mnt]# dd if=/dev/zero of=/mnt/storage/test.2 bs=1M count=300
> dd: error writing ‘/mnt/storage/test.2’: Input/output error
> 127+0 records in
> 126+0 records out
> 132120576 bytes (132 MB) copied, 14.2165 s, 9.3 MB/s
> From syslog:
> Apr 26 15:26:24 localhost lvm: WARNING: Thin pool vg_kvm-thinpool-tpool
> data is now 96.84% full.
> Apr 26 15:26:27 localhost kernel: device-mapper: thin: 253:4: reached low
> water mark for data device: sending event.
> Apr 26 15:26:27 localhost kernel: device-mapper: thin: 253:4: switching pool
> to out-of-data-space (error IO) mode
> Apr 26 15:26:34 localhost lvm: WARNING: Thin pool vg_kvm-thinpool-tpool
> data is now 100.00% full.
> Despite write errors, the filesystem is not in read-only mode:
But you get correct 'write' error - so from application POV - you get failing
transaction update/write - so app knows 'data' were lost and should not
proceed with next transaction - so it's in line with 'no data is lost' and
filesystem is not damaged and is in correct state (mountable).
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