Dne 12.9.2017 v 13:34 Gionatan Danti napsal(a):
On 12/09/2017 13:01, Zdenek Kabelac wrote:
There is very good reason why thinLV is fast - when you work with thinLV -
you work only with data-set for single thin LV.
Sad/bad news here - it's not going to work this way....
No, I absolutely *do not want* thinp to automatically dallocate/trash some
provisioned blocks. Rather, I all for something as "if free space is lower
than 30%, disable new snapshot *creation*"
# lvs -a
LV VG Attr LSize Pool Origin Data% Meta% Move Log
Cpy%Sync Convert
[lvol0_pmspare] vg ewi------- 2,00m
lvol1 vg Vwi-a-tz-- 20,00m pool 40,00
pool vg twi-aotz-- 10,00m 80,00 1,95
[pool_tdata] vg Twi-ao---- 10,00m
[pool_tmeta] vg ewi-ao---- 2,00m
[root@linux export]# lvcreate -V10 vg/pool
Using default stripesize 64,00 KiB.
Reducing requested stripe size 64,00 KiB to maximum, physical extent size
32,00 KiB.
Cannot create new thin volume, free space in thin pool vg/pool reached
threshold.
# lvcreate -s vg/lvol1
Using default stripesize 64,00 KiB.
Reducing requested stripe size 64,00 KiB to maximum, physical extent size
32,00 KiB.
Cannot create new thin volume, free space in thin pool vg/pool reached
threshold.
# grep thin_pool_autoextend_threshold /etc/lvm/lvm.conf
# Configuration option activation/thin_pool_autoextend_threshold.
# thin_pool_autoextend_threshold = 70
thin_pool_autoextend_threshold = 70
So as you can see - lvm2 clearly prohibits you to create a new thinLV
when you are above defined threshold.
To keep things single for a user - we have a single threshold value.
So what else is missing ?
lvm2 also DOES protect you from creation of new thin-pool when the fullness
is about lvm.conf defined threshold - so nothing really new here...
Maybe I am missing something: this threshold is about new thin pools or new
snapshots within a single pool? I was really speaking about the latter.
Yes - threshold applies to 'extension' as well as to creation of new thinLV.
(and snapshot is just a new thinLV)
Let me repeat: I do *not* want thinp to automatically drop anything. I simply
what it to disallow new snapshot/volume creation when unallocated space is too
low
as said - already implemented....
Committed (fsynced) writes are safe, and this is very good. However, *many*
application do not properly issue fsync(); this is a fact of life.
I absolutely *do not expect* thinp to automatically cope well with this
applications - I full understand & agree that application *must* issue proper
fsyncs.
Unfortunatelly lvm2 nor dm can be responsible for whole kernel logic and
all user-land apps...
Yes - anonymous pages cache is somewhat Achilles' heel - but it's not a
problem of thin-pool - all other 'provisioning' systems has some troubles....
So we really cannot fix it here.
You would need to prove that different strategy is better and fix linux kernel
for this.
Until this moment - you need use well written user-land apps :) properly
syncing written data - or not use thin-provisioning (and others).
You can also minimize amount of 'dirty' pages to avoid loosing too much data
in case you hit full thin-pool unexpectedly.....
You can sync every second to minimize amount of dirty pages....
Lots of things.... all of them will in some other the other impact system
performance....
In the past, I testified that XFS take its relatively long time to recognize
that a thin volume is unavailable - and many async writes can be lost in the
process. Ext4 + data=journaled did a better job, but a) it is not the default
filesystem in RH anymore and b) data=journaled is not the default option and
has its share of problems.
journaled is very 'secure' - but also very slow....
So depends what you aim for.
But this really cannot be solved on DM side...
So, if in the face of a near-full pool, thinp refuse me to create a new
filesystem, I would be happy :)
So you are already happy right :) ?
Your wish is upstream already for quite some time ;)
Regards
Zdenbek
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