Date: Sun, 11 Jan 2009 15:17:10 +1100 From: Peter Jeremy <peterjeremy@optushome.com.au> To: "M. Warner Losh" <imp@bsdimp.com> Cc: hackers@freebsd.org Subject: Re: 3x read to write ratio on dump/restore Message-ID: <20090111041710.GB5661@server.vk2pj.dyndns.org> In-Reply-To: <20090109.095027.-1672857892.imp@bsdimp.com> References: <20090109.095027.-1672857892.imp@bsdimp.com>
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--neYutvxvOLaeuPCA Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2009-Jan-09 09:50:27 -0700, "M. Warner Losh" <imp@bsdimp.com> wrote: >The read kBps was 3x the write kBps. =2E.. >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. 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). --=20 Peter Jeremy Please excuse any delays as the result of my ISP's inability to implement an MTA that is either RFC2821-compliant or matches their claimed behaviour. --neYutvxvOLaeuPCA Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (FreeBSD) iEYEARECAAYFAklpcsYACgkQ/opHv/APuIf68wCgq8Tr6rXQur5nEVBgVn4gPSLl O1AAnjf2Nn3jqnii5ZHq9BycVa6hFXe8 =C38A -----END PGP SIGNATURE----- --neYutvxvOLaeuPCA--
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