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Re: XFS over SSD

To: Raz <raziebe@xxxxxxxxx>
Subject: Re: XFS over SSD
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 15 Dec 2008 16:36:57 +1100
Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>, linux-xfs@xxxxxxxxxxx
In-reply-to: <5d96567b0812080711x34bb93d6vd8e4f88d9b190e9@xxxxxxxxxxxxxx>
Mail-followup-to: Raz <raziebe@xxxxxxxxx>, Eric Sandeen <sandeen@xxxxxxxxxxx>, linux-xfs@xxxxxxxxxxx
References: <5d96567b0812080442r131d9fc8t4019c99ffbffa290@xxxxxxxxxxxxxx> <493D334C.5010006@xxxxxxxxxxx> <5d96567b0812080711x34bb93d6vd8e4f88d9b190e9@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
On Mon, Dec 08, 2008 at 05:11:58PM +0200, Raz wrote:
> On Mon, Dec 8, 2008 at 4:46 PM, Eric Sandeen <sandeen@xxxxxxxxxxx> wrote:
> > Raz wrote:
> >> I am thinking of using XFS over a SSD disk.
> >> 1. Can I separate xfs meta data ( not just the logging) from the SSD ?
> >> can I put the meta on a different disk ?
> >
> > Are you talking about just the log (see the mkfs man page for external
> > logs, as Justin suggested) or all metadata?  For the latter, using the
> > realtime subvolume does accomplish this (data on one volume, metadata on
> > the other) but that's not used very often.
> >
> > -Eric
> >
> I am referring to all the meta data. 128K of erase block for some block map
> update is a big penalty.

That's not an issue for smart SSDs like the intel one. They
internally use a log-based structure that means that small writes
don't end up requiring an entire erase block to be rewritten.  That
is, they have a low write amplification factor.

Such SSDs don't really care that much about the filesystem structure
and random write patterns. Older/cheaper SSDs have a much larger
write amplification factor and so are unsuited to traditional
filesystem structures.....

Google "SSD write amplification" for more information.


Dave Chinner

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