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Date: Tue, 09 Jul 2013 20:47:06 +0200
From: Zdenek Kabelac <zkabelac@redhat.com>
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Subject: Re: [linux-lvm] Very slow i/o after snapshotting
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To: LVM general discussion and development <linux-lvm@redhat.com>
Cc: Marian Csontos <mcsontos@redhat.com>, Micky <mickylmartin@gmail.com>

Dne 9.7.2013 17:39, Micky napsal(a):
> I meant alignment for all dm entries 0 through 31 is zero!
>
>
>>> What does the `lsblk -t` say? Could be an alignment issue.
>>
>> 0 through 31
>>
>>> 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)
>>
>> Cached mem looks good.
>> Dmeventd. Right. It is. Isn't it spawed everytime an LV is created?
>> root      6813  0.0  1.4 197056 11044 ?        S<s  May26   2:44 /sbin/dmeventd
>

There is only one dmeventd running - and lvm is spawning it only when it's not
available - and in fact spawning is not the right term if you use systemd 
enabled system (like Fedora)

Also so far you still have not show actually any 'real' numbers even when you 
run plain good old  'dd' command.

So what is the performance of 'dd'  reading 10G >/dev/null
or raw device,  dm origin, dm snapshot  (with  iflag=direct)

What is performance of write ?

What is the performance when 2 of them are running in parallel.

Also it's probably more easier to resolve this through #irc.

Zdenek