Messages in this thread | | | Date | Tue, 25 Aug 2009 18:19:40 -0700 (PDT) | From | david@lang ... | Subject | Re: [patch] document flash/RAID dangers |
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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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