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SubjectRe: soft update vs journaling?
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James Courtier-Dutton wrote:
> John Richard Moser wrote:
>> Unfortunately, journaling uses a chunk of space. Imagine a journal on a
>> USB flash stick of 128M; a typical ReiserFS journal is 32 megabytes!
>> Sure it could be done in 8 or 4 or so; or (in one of my file system
>> designs) a static 16KiB block could reference dynamicly allocated
>> journal space, allowing the system to sacrifice performance and shrink
>> the journal when more space is needed. Either way, slow media like
>> floppies will suffer, HARD; and flash devices will see a lot of
>> write/erase all over the journal area, causing wear on that spot.
> My understanding is that if one designed a power supply with enough
> headroom, one could remove the power and still have time to write dirty
> sectors to the USB flash stick. Would this not remove the need for a
> journaling fs on a flash stick.

Depends on how much meta-data you have to write out. What if you just
altered 6000 files? Now you have a ton of dentries to destroy and
inodes to invalidate, some FAT entries to free up, etc. What if the
user just pulled the drive out of the USB port? Or the USB port is
faulty and lost connection (I've seen it!).

> This would remove the "wear on that
> spot" problem.

Wha? You mean remove the trigger, not the underlying problem.

> Actually USB flash sticks are a bit clever, in that they
> add an extra layer of translation to the write. I.e. If you write to the
> same sector again and again, the USB flash stick will actually write it
> to a different area of the memory each time. This is specifically done
> to save the "wear on that spot" problem.

Yeah, built-in write balancing is nice.

> This "flush on power fail" approach is not so easy with a HD because it
> uses more power and takes longer to flush.

The "flush on power fail" is retarded because it takes extra hardware
and doesn't work if the USB port itself loses connection or if the user
is just dumb enough to pull/knock the drive out. It won't work with
mini hard disks either, as you say.

"Flush on power fail" is pretty much getting a 10 minute UPS and issuing
'shutdown -h now' when the UPS signals init, which there's already
contingencies for (can also suspend to disk). It won't help if the PSU
burns out, if the system crashes, if the power cord is pulled, or if the
dog walks around your chair and you turn and bump your foot into the
"power" button on the UPS itself.

> James

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