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Re: [linux-lvm] Very slow i/o after snapshotting



Dne 9.7.2013 16:04, Micky napsal(a):
Do you write to the snapshot ?

Not so often but there is like 1-5% usage allocation.

It's known FACT that performance of old snapshot is very far from being
ideal - it's very simply implementation - for a having consistent system to
make a backup of the volume - so for backup it doesn't really matter how
slow is that (it just needs to remain usable)

True. But in case of domains running on a hypervisor, the purpose of doing
a live backup slingshots and dies! I know it's not LVM's fault but
sluggishness is!

Well here we are at lvm list - thus discussing lvm metadata and command line issues - do you see slow command line execution ?

I think you are concerned about the perfomance of dm device - which
is a level below lvm  (kernel level)

Do not take is as some excuse - just we should use correct terms.



I'd suggest to go with much smaller chunks - i.e. 4-8-16KB - since if you
update a single 512 sector  -  512KB of data has to be copied!!! so really
bad idea, unless you overwrite large continuous portion of a device.

I just tried that and got 2-3% improvement.
Here are the gritty details, if someone's interested.
   --- Logical volume ---
   LV Write Access        read/write
   LV snapshot status     active destination for lvma
   LV Status              available
   # open                 1
   LV Size                200.10 GiB
   Current LE             51226
   COW-table size         100.00 GiB

Well here is the catch I guess.

While the snapshot might be reasonable enough with sizes like 10GiB,
it's getting much much worse when it scales up.

If you intent to use  100GiB snapshot - please consider thin volumes here.
Use upstream git and report bugs if something doesn't work.
There is not going to be a fix for old-snaps - the on-disk format it quite unscalable. Thin is the real fix for your problems here.
Also note - you will get horrible start-up times for snapshot of this size...


And yes - if you have rotational hdd - you need to expect horrible seek
times as well when reading/writing from snapshot target....

Yes, they do. But I reproduced this one with multiple machines (and kernels)!

Once again - there is no hope  old-snaps could become magically faster unless
completely rewritten - and that what's thin provisioning is basically about ;)
We've tried to make everything much faster and smarter.
So do not ask for fixing old snapshots - they are simply unfixable for large
COW sizes - it's been designed for something very different then you try to use it for...


And yes - there are some horrible Segate hdd drives (as I've seen just
yesterday) were 2 disk reading programs at the same time may degrade 100MB/s
-> 4MB/s (and there is no  dm involved)

Haha, no doubt. Seagates' are the worst ones. IMHO, Hitachi's drives
run cooler and
that's what Nagios tells me!

Just simple check is how fast parallel 'dd' you get from /dev/sda partition - if you get approximately halve the speed of single 'dd' - then you have good enough drive (Hitachi is usually pretty good).

Zdenek


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