On Fri, 3 Nov 2006, Mikulas Patocka wrote:
On Thu, 2 Nov 2006, Mikulas Patocka wrote:
> As my PhD thesis, I am designing and writing a filesystem, and it's now
> in a state that it can be released. You can download it from
> http://artax.karlin.mff.cuni.cz/~mikulas/spadfs/
"Disk that can atomically write one sector (512 bytes) so that the sector
contains either old or new content in case of crash."
Well, maybe I am completly wrong but as far as I understand no disk
currently will provide such requirement. Disks can have (after halted
write):
- old data,
- new data,
- nothing (unreadable sector - result of not full write and disk internal
checksum failute for that sector, happens especially often if you have
frequent power outages).
And possibly some broken drives may also return you something that they
think is good data but really is not (shouldn't happen since both disks
and cables should be protected by checksums, but hey... you can never be
absolutely sure especially on very big storages).
So... isn't this making your filesystem a little flawed in design?
There was discussion about it here some times ago, and I think the result was
that the IDE bus is reset prior to capacitors discharge and total loss of
power and disk has enough time to finish a sector --- but if you have crap
power supply (doesn't signal power loss), crap motherboard (doesn't reset
bus) or crap disk (doesn't respond to reset), it can fail.
Hmm, maybe. But I think I saw couple of such bad sectors that were only
bad because of power loss in the wild.
BTW. reiserfs and xfs depend on this feature too. ext3 is the only one that
doesn't.
Well, at least for XFS everybody tell that it should be used with UPS only
if you really care about your data. I think it has something to do with
heavy in-RAM caching this filesystem does.
Anyway, it looks strange to list something very fragile and potentially
not existing in the requirements... :-)
Could you explain where exactly do you depend on this requirement? And
what could happen if it is not true?
Thanks,
Grzegorz Kulewski
PS. Do you have any benchmarks of your filesystem? Did you do any longer
automated tests to prove it is not going to loose data to easily?
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