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Re: XFS Preallocate using ALLOCSP

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: XFS Preallocate using ALLOCSP
From: Smit Shah <getsmit@xxxxxxxxx>
Date: Tue, 16 Jun 2009 14:38:12 -0700
Cc: Felix Blyakher <felixb@xxxxxxx>, linux-xfs@xxxxxxxxxxx
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In the vmware sparse disk,  the chunks are allocated of the size of 64k,
which kindof leads to fragmentation. Hence the sequential read throughput
suffers. So thought it might be a good idea to preallocate some
amount memory say 50Meg when it asks for allocating the chunk thus helps
in reducing fragmentation and improves the throughput.
On linux i do preallocation using fallocate.

I have vmware workstation running Windows Vista and host is RHEL 5.3
with the guest having 1GB of memory and host having 2 Gig. And i have
separate dedicated SATA disk on which i perform the tests. For
testing, I create a new sparse vdisk and run a IOmeter sequential
write on it.

For ext4 i tend to get say around 5-6 Mbps of improvement in write
throughput sometimes but it never goes down. But for XFS i always see
8-10 Mbps of downfall.


On 6/16/09, Eric Sandeen <sandeen@xxxxxxxxxxx> wrote:
> Smit Shah wrote:
>> On 6/16/09, Felix Blyakher <felixb@xxxxxxx> wrote:
>>> On Jun 16, 2009, at 10:16 AM, Eric Sandeen wrote:
>>>> Maybe I should ask what the end goal is here.  :)
>> Just to see if preallocation using fallocate helps reducing the
>> fragmentation and increases the throughput.  I guess it wll help
>> reduce the fragmentation but the write performance is going to suffer.
> Depends on the IO you do, I think.  I'd be curious to know exactly how
> you're running preallocation & iometer so I could give it a shot, to see
> why xfs seems to be doing worse than ext4.

> Thanks,
> -Eric

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