linux-ext4 - Re: question about file space preallocation with fallocate

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Date:	Mon, 27 Dec 2010 17:57:21 +0000
From:	Filipe David Manana <fdmanana@...che.org>
To:	Sunil Mushran <sunil.mushran@...cle.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: question about file space preallocation with fallocate

On Mon, Dec 27, 2010 at 5:17 PM, Sunil Mushran <sunil.mushran@...cle.com> wrote:
>
> fallocate() gives users the ability to allocate space instantly. One way
> to compare would be to time just fallocate() with another program
> writing zeros for that length.
>
> But that's not the aim of the syscall. The aim is to allow the fs to
> allocate
> the space in as large chunks as possible to allow for better read
> performance.
>
> If you don't do fallocate() and allow writes to allocate in small chunks,
> as you are doing, the allocations on disks could be interleaved in face of
> multiple processes doing the same. Fragmented allocations can only hurt
> read performance.
>

Thanks for the clarification Sunil. But preallocation of blocks
shouldn't also improve write operations? Since each write operation
will no longer cause the OS/filesystem to allocate blocks for the
file, therefore should be faster.

Also, any particular advice for improving write performance when all
the writes are done in append-only fashion?

-- 
Filipe David Manana,
fdmanana@...il.com, fdmanana@...che.org

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."
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