Re: [linux-lvm] Thin pool performance when allocating lots of blocks

From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: Zdenek Kabelac <zdenek.kabelac@gmail.com>,
	LVM general discussion and development <linux-lvm@redhat.com>,
	device-mapper development <dm-devel@redhat.com>
Cc: "Frédéric Pierret" <frederic.pierret@qubes-os.org>,
	"Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
Subject: Re: [linux-lvm] Thin pool performance when allocating lots of blocks
Date: Tue, 8 Feb 2022 16:02:40 -0500	[thread overview]
Message-ID: <733f87f3-5ed9-b266-b951-4526f872bad1@invisiblethingslab.com> (raw)
In-Reply-To: <9a14a7a5-a8a1-a4d6-f9fd-012d3c170f2a@gmail.com>

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On 2/8/22 15:37, Zdenek Kabelac wrote:
> Dne 08. 02. 22 v 20:00 Demi Marie Obenour napsal(a):
>> Are thin volumes (which start as snapshots of a blank volume) efficient
>> for building virtual machine images?  Given the nature of this workload
>> (writing to lots of new, possibly-small files, then copying data from
>> them to a huge disk image), I expect that this will cause sharing to be
>> broken many, many times, and the kernel code that breaks sharing appears
>> to be rather heavyweight.  Furthermore, since zeroing is enabled, this
>> might cause substantial write amplification.  Turning zeroing off is not
>> an option for security reasons.
>>
>> Is there a way to determine if breaking sharing is the cause of
>> performance problems?  If it is, are there any better solutions?
> 
> Hi
> 
> Usually the smaller the thin chunks size is the smaller the problem gets.
> With current released version of thin-provisioning minimal chunk size is 
> 64KiB. So you can't use smaller value to further reduce this impact.
> 
> Note - even if you do a lot of tiny 4KiB writes  - only the 'first' such write 
> into 64K area breaks sharing all following writes to same location no longer 
> have this penalty (also zeroing with 64K is less impactful...)
> 
> But it's clear thin-provisioning comes with some price - so if it's not good 
> enough from time constrains some other solutions might need to be explored.
> (i.e. caching, better hw, splitting  FS into multiple partitions with 
> 'read-only sections,....)

Are the code paths that break sharing as heavyweight as I was worried
about?  Would a hypothetical dm-thin2 that used dm-bio-prison-v2 be
faster?

-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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