Re: number of subvolumes

[Ferry Toth <ftoth@telfort.nl>]

Op Wed, 23 Aug 2017 10:37:07 +0200, schreef A L:

> ---- From: Ulli Horlacher <framstag@rus.uni-stuttgart.de> -- Sent:
> 2017-08-23 - 09:18 ----
> 
>> On Tue 2017-08-22 (22:48), Ulli Horlacher wrote:
>> 
>>> > Assumptions that all Btrfs features such as snapshots are infinitely
>>> > scalable at no cost may be optimistic:
>>> > 
>>> >   https://btrfs.wiki.kernel.org/index.php/
Gotchas#Having_many_subvolumes_can_be_very_slow
>>> 
>>> "when you do device removes on file systems with a lot of snapshots,
>>> it
>>>  is unbelievably slow ... took nearly a week to move 20GB of FS data
>>>  from one device to the other using that method"
>>>   
>>> "a balance on 2TB of data that was heavily snapshotted - it took 3
>>> months"
>> 
>> This is a vanilla SLES12 installation:
>> 
>> root@ptm1:~# grep PRETTY_NAME /etc/os-release PRETTY_NAME="SUSE Linux
>> Enterprise Server 12 SP1"
>> 
>> root@ptm1:~# btrfs subvolume list /
>> ID 257 gen 358277 top level 5 path @
>> ID 258 gen 978361 top level 257 path @/home ID 259 gen 1252501 top
>> level 257 path @/opt ID 260 gen 883012 top level 257 path @/srv ID 261
>> gen 1252673 top level 257 path @/tmp ID 262 gen 1252501 top level 257
>> path @/usr/local ID 263 gen 882958 top level 257 path @/var/crash ID
>> 264 gen 1252673 top level 257 path @/var/log ID 265 gen 882923 top
>> level 257 path @/var/opt ID 266 gen 1252673 top level 257 path
>> @/var/spool ID 267 gen 1252668 top level 257 path @/var/tmp ID 270 gen
>> 1252668 top level 257 path @/.snapshots ID 452 gen 358277 top level 270
>> path @/.snapshots/127/snapshot ID 453 gen 1252670 top level 270 path
>> @/.snapshots/128/snapshot ID 540 gen 368554 top level 270 path
>> @/.snapshots/191/snapshot ID 542 gen 419566 top level 270 path
>> @/.snapshots/192/snapshot ID 1035 gen 1027889 top level 270 path
>> @/.snapshots/539/snapshot ID 1036 gen 1027889 top level 270 path
>> @/.snapshots/540/snapshot ID 1045 gen 1048327 top level 270 path
>> @/.snapshots/545/snapshot ID 1046 gen 1048327 top level 270 path
>> @/.snapshots/546/snapshot ID 1062 gen 1068800 top level 270 path
>> @/.snapshots/555/snapshot ID 1063 gen 1068800 top level 270 path
>> @/.snapshots/556/snapshot ID 1122 gen 1130369 top level 270 path
>> @/.snapshots/595/snapshot ID 1123 gen 1130369 top level 270 path
>> @/.snapshots/596/snapshot ID 1124 gen 1171229 top level 270 path
>> @/.snapshots/597/snapshot ID 1125 gen 1171229 top level 270 path
>> @/.snapshots/598/snapshot ID 1135 gen 1171229 top level 270 path
>> @/.snapshots/605/snapshot ID 1136 gen 1171229 top level 270 path
>> @/.snapshots/606/snapshot ID 1137 gen 1171229 top level 270 path
>> @/.snapshots/607/snapshot ID 1138 gen 1171229 top level 270 path
>> @/.snapshots/608/snapshot ID 1139 gen 1171229 top level 270 path
>> @/.snapshots/609/snapshot ID 1140 gen 1171229 top level 270 path
>> @/.snapshots/610/snapshot ID 1141 gen 1171229 top level 270 path
>> @/.snapshots/611/snapshot ID 1142 gen 1171229 top level 270 path
>> @/.snapshots/612/snapshot ID 1158 gen 1172970 top level 270 path
>> @/.snapshots/613/snapshot ID 1159 gen 1172972 top level 270 path
>> @/.snapshots/614/snapshot
>> 
>> Why does SUSE ignore this "not too many subvolumes" warning?
> 
> Using hundreds or thousands of snapshots is probably fine mostly.
> perhaps the slow performance is more related to what changed between
> them? I have regularly that many snapshots and export many as "Previous
> Versions" to Windows clients over Samba without any performance issues.
> But my data doesn't change that much.
> 
> I think those comments on the Wiki are a little misleading without
> better details to what workloads are affected this way.
> Perhaps someone can set up some tests and publish the results?
> 

We find that typically apt is very slow on a machine with 50 or so 
snapshots and raid10. Slow as in probably 10x slower as doing the same 
update on a machine with 'single' and no snapshots.

Other operations seem to be the same speed, especially disk benchmarks do 
not seem to indicate any performance degradation.

>> 
>> --
>> Ullrich Horlacher              Server und Virtualisierung Rechenzentrum
>> TIK Universitaet Stuttgart         E-Mail:
>> horlacher@tik.uni-stuttgart.de Allmandring 30a                Tel:   
>> ++49-711-68565868 70569 Stuttgart (Germany)      WWW:   
>> http://www.tik.uni-stuttgart.de/
>> REF:<20170822204811.GO14804@rus.uni-stuttgart.de>
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