On 2011-Oct-18 23:18:02 +1100, Edward Ned Harvey 
<opensolarisisdeadlongliveopensola...@nedharvey.com> wrote:
>I recently put my first btrfs system into production.  Here are the
>similarities/differences I noticed different between btrfs and zfs:

Thanks for that.

>* zfs has storage tiering.  (cache & log devices, such as SSD's to
>accelerate performance.)  btrfs doesn't have this yet.

I'd call that "multi-level caching and journalling".  To me, storage
tiering means something like HSM - something that lets me push rarely
used data to near-line storage (eg big green SATA drives that are spun
down most of the time) whilst retaining the ability to transparently
access it.

On 2011-Oct-19 03:46:30 +1100, Mark Sandrock <mark.sandr...@oracle.com> wrote:
>Doesn't a scrub do more than what 'fsck' does?

It does different things.  I'm not sure about "more".

fsck verifies the logical consistency of a filesystem.  For UFS, this
includes: used data blocks are allocated to exactly one file,
directory entries point to valid inodes, allocated inodes have at
least one link, the number of links in an inode exactly matches the
number of directory entries pointing to that inode, directories form a
single tree without loops, file sizes are consistent with the number
of allocated blocks, unallocated data/inodes blocks are in the
relevant free bitmaps, redundant superblock data is consistent.  It
can't verify data.

scrub uses checksums to verify the contents of all blocks and attempts
to correct errors using redundant copies of blocks.  This implicitly
detects some types of logical errors.  I don't know if scrub includes
explicit logic to detect things like directory loops, missing free
blocks, unreachable allocated blocks, multiply allocated blocks, etc.

>IIRC, fsck was seldom needed at
>my former site once UFS journalling
>became available. Sweet update.

Whilst Solaris very rarely insists we run fsck, we have had a number
of cases where we have found files corrupted following a crash - even
with UFS journalling enabled.  Unfortunately, this isn't the sort of
thing that fsck could detect.

Peter Jeremy

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