On Wed, 26 Aug 2009, Pavel Machek wrote:
THESE devices have the property of potentially corrupting blocks being
written at the time of the power failure,
this is true of all devices
Actually I don't think so. I believe SATA disks do not corrupt even
the sector they are writing to -- they just have big enough
capacitors. And yes I believe ext3 depends on that.
Pavel, no S-ATA drive has capacitors to hold up during a power failure
(or even enough power to destage their write cache). I know this from
direct, personal knowledge having built RAID boxes at EMC for years. In
fact, almost all RAID boxes require that the write cache be hardwired to
off when used in their arrays.
I never claimed they have enough power to flush entire cache -- read
the paragraph again. I do believe the disks have enough capacitors to
finish writing single sector, and I do believe ext3 depends on that.
keep in mind that in a powerfail situation the data being sent to the
drive may be corrupt (the ram gets flaky while a DMA to the drive copies
the bad data to the drive, which writes it before the power loss gets bad
enough for the drive to decide there is a problem and shutdown)
you just plain cannot count on writes that are in flight when a powerfail
happens to do predictable things, let alone what you consider sane or
proper.
David Lang
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