Date: Thu, 20 Feb 1997 05:55:07 -0500 (EST) From: Peter Dufault <dufault@hda.com> To: asami@vader.cs.berkeley.edu (Satoshi Asami) Cc: joerg_wunsch@uriah.heep.sax.de, james@wgold.demon.co.uk, hackers@FreeBSD.org, dufault@hda.com Subject: Re: drive write caches Message-ID: <199702201055.FAA02640@hda.hda.com> In-Reply-To: <199702200052.QAA00662@silvia.HIP.Berkeley.EDU> from Satoshi Asami at "Feb 19, 97 04:52:38 pm"
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> * > - if the OS tells me it has a successful write through the
> * > disk device, can I rely on:
> * >
> * > a) the data is on the platter
> *
> * It is. Since physio(9) waits for the completion, you should be
> * notified of any lower-level errors. Note that this is _unlike_ with
> * the block devices, where writing happens asynchronously, so the kernel
> * did already declare the write operation as successful by the time it
> * notices an error.
>
> Really? I think drives have their own cache, and if they have write
> buffering turned on, they will happily return "write complete" when
> they are only holding data in the buffer.
This is the deferred error that you can get a system console message
about - it means there was an error for a transfer that already
returned a successful completion. I believe the way to handle this properly
is to issue a SYNCHRONIZE CACHE command at the right place and then
handle any deferred errors that come back as a result. This
would require putting a hook in the file system code someplace
to do that sync / fixup.
The page to turn it off is the caching page (page 8):
# Caching page:
0x08 {
{Reserved} *t5
{WCE} t1
{MF} t1
{RCD} t1
{Demand Retention Priority} t4
{Write Retention Priority} t4
{Disable Pre-fetch Transfer Length} i2
{Minumum Pre-fetch} i2
{Maximum Pre-fetch} i2
{Maximum Pre-fetch Ceiling} i2
}
WCE is write cache enable. I've just checked and two of my disks
(from Rodney, Rod, if you're out there) have the write cache disabled.
Peter
--
Peter Dufault (dufault@hda.com) Realtime Machine Control and Simulation
HD Associates, Inc. Voice: 508 433 6936
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