Date: Sat, 10 Jan 2009 22:03:22 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: peterjeremy@optushome.com.au Cc: hackers@FreeBSD.org Subject: Re: 3x read to write ratio on dump/restore Message-ID: <20090110.220322.2008390173.imp@bsdimp.com> In-Reply-To: <20090111041710.GB5661@server.vk2pj.dyndns.org> References: <20090109.095027.-1672857892.imp@bsdimp.com> <20090111041710.GB5661@server.vk2pj.dyndns.org>
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In message: <20090111041710.GB5661@server.vk2pj.dyndns.org>
Peter Jeremy <peterjeremy@optushome.com.au> writes:
: On 2009-Jan-09 09:50:27 -0700, "M. Warner Losh" <imp@bsdimp.com> wrote:
: >The read kBps was 3x the write kBps.
: ...
: >Any ideas what gives? I observed this with 16MB cache and with 32MB
: >cache, fwiw.
:
: I've seen this as well. AFAIK, this is a side-effect of dump's caching.
:
: My top-of-head explanation is that each dump process has its own cache
: but actual I/O is round-robined on a (roughly) block scale so a large
: contiguous file will wind up in each 'slave' process's cache.
:
: The most obvious (and easiest) fixes are to either implement a shared
: cache (though this means another level of inter-process communication)
: or only use a single 'slave' process when caching is enabled.
I'll have to look into this... Most of the files I was backing up
were large contiguous files (26 10GiB .dv files from my camera).
: The cache algorithm could probably be enhanced as well - apart from
: inode blocks, any block will only be accessed once so once a block has
: been accessed, it can be purged from the cache (which is completely
: opposite to a "normal" cache).
Yes, read everything, purge once it is touched.
Warner
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