ATA write cache

[Divacky Roman]

On Wed, Jun 02, 2004 at 05:00:14PM +0200, Radim Kolar wrote:
> zda se ze to neni moc dobry napad. V linuxu je defaultne off.
 
1) kdo "tvrde" resetuje pocitac tak si koleduje o problemy tak jako tak
(proto mame UPSky apod.)

2) z toho co ja vim tak primarni je zachovani konzistence fs ne dat (k
tomu jsou zalohy). a dost pochybuju ze by v te cache byla "pulka
transakce"

3) v fbsd to taky driv bylo off ale pak to zapli (pote co jsou
softupdates defaultne on), takze si myslim ze to bylo dobre zvazeno

ja osobne jsem s tim nemel nikdy problem
 
> =On May 20, 2004, at 4:15 PM, Mikhail Teterin wrote:
> => =No. Unlike SCSI disks, ATA disks will toss their write-cache on a
> => =reset. When the system crashes and the BIOS starts rebooting, guess
> => =what it issues to the ATA disks? Yep, a reset.
> =
> =I checked with my BIOS source, and I got this wrong. After a crash, the
> =BIOS is usually entered as a result of the crash-code (panic(), in the
> =case of *BSD) invoking a hard reset by writing to the reset bit of I/O
> =port 92. This causes a reset of the hardware, which includes the PCI
> =bus and the ATA disks as well as the CPU.
> 
> => So with ATA write-cache =enabled, your filesystem is likely to be
> => toast after a crash, as well =as after a power failure.
> =>
> => Is not this only of concern if the power is restored and the BIOS
> => resets the disks _before_ they flush their write caches? I'd expect
> => them to do that (the flushing) within seconds anyway, no?
> 
> =Yes, if the disks manage to win the race by flushing their write cache
> =before the hard reset is issued by the crash code, then everything is
> =fine. [...] I think the panic code quite soon after getting control,
> =and the disks flush their write cache rather lazily, rather than
> =eagerly.
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