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To: arch@freebsd.org
From: Poul-Henning Kamp <phk@phk.freebsd.dk>
Date: Fri, 28 Mar 2008 19:09:28 +0000
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Subject: Flash disks and FFS layout heuristics
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I've laid my hands on a "M-Tron MOBI3000 32GB" flash disk (2.5" format,
it'll be in my laptop before long :-)

Here is a naive benchmark sequence, comparing it to a WD Raptor
(<WDC WD360GD-00FNA0 35.06K35>)

                               Flash            Disk		
---------------------------------------------------------------
Empty fsck:                        0.83            2.47    -66%
restore -rf                      839            1251	   -33%
loaded fsck:                      10.34           78.81	   -87%
dump 0f /dev/null:               563.21         1094.91	   -49%
---------------------------------------------------------------

So far so good, it's clearly the seektime that dominates the
flash-advantage.

But this reproducible observation by fsck concerns me a bit:

   Flash:  (205727 frags,  896270 blocks, 1.4% fragmentation)

   Disk:   (197095 frags, 1193944 blocks, 1.1% fragmentation)

I might indicate that the flash is fast enough to confuse some of
FFS's layout heuristics.

Any aspiring filesystems hackers should start to consider the
implications for filesystemlayout, if there is in essence no
seek-time penalty for reads and a fair seek pentalty for writes.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.