Date:	Thu, 23 Aug 2007 13:56:17 +0100
From:	"Daniel J Blueman" <daniel.blueman@...il.com>
To:	"Fengguang Wu" <wfg@...l.ustc.edu.cn>
Cc:	"Jan Engelhardt" <jengelh@...putergmbh.de>,
	"Richard Ballantyne" <richardballantyne@...il.com>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>,
	"James Courtier-Dutton" <James@...erbug.co.uk>
Subject: Re: file system for solid state disks

Hi Fengguang,

On 23/08/07, James Courtier-Dutton <James@...erbug.co.uk> wrote:
> Daniel J Blueman wrote:
> > On 23 Aug, 07:00, Jan Engelhardt <jengelh@...putergmbh.de> wrote:
> >> On Aug 23 2007 01:01, Richard Ballantyne wrote:
> >>
> >>> What file system that is already in the linux kernel do people recommend
> >>> I use for my laptop that now contains a solid state disk?
> >>>
> >> If I had to choose, the list of options seems to be:
> >>
> >> - logfs
> >>   [unmerged]
> >>
> >> - UBI layer with any fs you like
> >>   [just a guess]
> >>
> >> - UDF in Spared Flavor (mkudffs --media-type=cdrw --utf8)
> >>   [does not support ACLs/quotas]
> >
> > Isn't it that with modern rotational wear-levelling, re-writing hot
> > blocks many times is not an issue, as they are internally moved around
> > anyway? So, using a journalled filesystem such as ext3 is still good
> > (robustness and maturity in mind). Due to lack of write buffering,
> > perhaps a wandering log (journal) filesystem would be more suitable
> > though? I use ext3 on my >35MB/s compact flash filesystem.
> >
> > I can see there being advantage in selecting a filesystem which is
> > lower complexity due to no additional spatial optimisation complexity,
> > but those advantages do buy other efficiency (eg the Orlov allocator
> > reducing fragmentation, thus less overhead), right?
> >
> > Also, it would be natural to employ 'elevator=none', but perhaps there
> > is a small advantage in holding a group of flash blocks 'ready' (like
> > SDRAM pages being selected on-chip for lower bus access latency) -
> > however this no longer holds when logical->physical remapping is
> > performed, so perhaps it's better without an elevator.
> >
> > Clearly, benchmarks speak...but perhaps it would make sense to have
> > libata disable the elevator for the (compact) flash block device?
> >
> > Daniel
>
> Also, sector read ahead will actually have a performance impact on
> Flash, instead of speed things up with a spinning disc.
> For example, a request might read 128 sectors instead of the one
> requested at little or no extra performance impact for a spinning disc.
> For flash, reading 128 sectors instead of the one requested will have a
> noticeable performance impact.
> Spinning discs have high seek latency, low serial sector read latency
> and equal latency for read/write
> Flash has low seek latency, high serial sector read latency and longer
> write than read times.

I was having problem invoking the readahead logic on my compact flash
rootfs (ext3) with tweaking the RA with 'hdparm -a' from 8 to 1024
blocks and some benchmarks (I forget which).

Fengguang, what is your favourite benchmark for finding differences in
readahead values (running on eg ext3 on a flashdisk), with the current
RA semantics in mainline kernels (eg 2.6.23-rc3)?

Thanks,
  Daniel
-- 
Daniel J Blueman
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