good to hear

Posted Jun 17, 2007 21:43 UTC (Sun) by joern (guest, #22392)
In reply to: good to hear by qu1j0t3
Parent article: Linus on GPLv3 and ZFS

> From comments I've heard, and the buzz around Linus' and Jon's posts, there seems to be considerable community interest around ZFS (I don't want to use anything else, or wait, so I switched to Solaris 10 some time ago).

> Look forward to more news on this front. Did you follow-up on LKML? (I have not checked :)

My personal interest is in flash, not hard disks. Therefore ZFS is impressive technology, but solving someone else's problem. It is not the last word in filesystems either, as the fsck will run for hours or days if it ever becomes necessary. So there remain valid reasons to work on different filesystems.

Impressive technology none the less.

to post comments

COW for Flash?

Posted Jun 18, 2007 17:26 UTC (Mon) by qu1j0t3 (guest, #25786) [Link] (1 responses)

It has been said that COW is ideal for Flash. Can you explain why ZFS isn't relevant here?

There is no fsck; ZFS is "always consistent on disk" (through COW+atomic transactions). It seems to me this is a necessary invariant to achieve its other features (such as snapshots). Debate flares up (occasionally) as to whether a scavenger will be necessary. If so, it won't much resemble 'fsck' - and certainly won't be run in normal operation or after reset/powerfail/etc (ZFS behaviour under impromptu reset is extremely well tested).

I suspect, but correct me if I'm wrong, once you "know" you've lost data in ZFS (through exhausting redundancy or ditto blocks), it's actually gone by definition, and unrecoverable by re-creating links. No doubt Bonwick et all have explained it better somewhere...

COW for Flash?

Posted Jun 19, 2007 8:17 UTC (Tue) by joern (guest, #22392) [Link]

> It has been said that COW is ideal for Flash. Can you explain why ZFS isn't relevant here?

Raw flash behaves sufficiently different to hard disks that some ZFS design assumptions become untrue. Flash has large erase blocks. Within erase blocks, data must be written from front to back. Writing the block again requires erasing all of it. So the filesystem block size either has to be equal to the erase block size, or you need garbage collection. And with garbage collection comes a nasty deadlock problem most people don't even realize exists. :)

Next comes wear out and protection against it. Afaics, ZFS has several hot zones that receive significantly more writes than others.

I guess those two are the big reasons.