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

[Marian Csontos]

On 07/09/2013 04:57 PM, Micky wrote:
> Ahh. I get it. Sorry for using the aging old snap mechanism. Seems no
> more luck with it now! I'll have to test the Thin in such an
> environment to have my say. But not gonna try it anytime soon. The
> power pill I am being referred to has sadly no recovery options ;)
> Thanks for the suggestions though!

You could still try to allocate the snapshot on another HDD if possible 
to see if HDD seektime may be the issue. Still the performance should 
not be degraded that drastically.

What does the `lsblk -t` say? Could be an alignment issue.

What's `free` saying about the free memory and cache? (dmeventd on 6.4 
is trying to lock a large chunk of address space in RAM (~100M)

-- Marian

>
> On Tue, Jul 9, 2013 at 7:18 PM, Zdenek Kabelac <zkabelac at redhat.com> wrote:
>> 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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