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Re: SSD + RAID

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: SSD + RAID
Date: 2010-02-22 14:39:55
Message-ID: 201002221439.o1MEdth13439@momjian.us (view raw or flat)
Thread:
Lists: pgsql-performance
Ron Mayer wrote:
> Bruce Momjian wrote:
> > Agreed, thought I thought the problem was that SSDs lie about their
> > cache flush like SATA drives do, or is there something I am missing?
> 
> There's exactly one case I can find[1] where this century's IDE
> drives lied more than any other drive with a cache:
> 
>   Under 120GB Maxtor drives from late 2003 to early 2004.
> 
> and it's apparently been worked around for years.
> 
> Those drives claimed to support the "FLUSH_CACHE_EXT" feature (IDE
> command 0xEA), but did not support sending 48-bit commands which
> was needed to send the cache flushing command.
> 
> And for that case a workaround for Linux was quickly identified by
> checking for *both* the support for 48-bit commands and support for the
> flush cache extension[2].
> 
> 
> Beyond those 2004 drive + 2003 kernel systems, I think most the rest
> of such reports have been various misfeatures in some of Linux's
> filesystems (like EXT3 that only wants to send drives cache-flushing
> commands when inode change[3]) and linux software raid misfeatures....
> 
> ...and ISTM those would affect SSDs the same way they'd affect SATA drives.

I think the point is not that drives lie about their write-back and
write-through behavior, but rather that many SATA/IDE drives default to
write-back, and not write-through, and many administrators an file
systems are not aware of this behavior.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com
  PG East:  http://www.enterprisedb.com/community/nav-pg-east-2010.do
  + If your life is a hard drive, Christ can be your backup. +

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