There's been some talk about alignment lately, both for flash and WD disks.
What's missing, at least from my perspective, is a clear an unambiguous test so users can verify that their zfs pools are aligned correctly. This should be a test that sees through all the layers of BIOS and SMI/EFI and zfs labels and their accumulated offsets, and lets us ascertain where things land in terms of addresses that matter to the storage. I can think of two methods to achieve this, but lack information to complete either. I'd appreciate some help - or a better way. #1. Use xxd (or similar) to examine the contents of the raw disk device that ignores partitioning (e.g. c0d0p0). Search for a known label magic number or similar content, and determine its address. Apply arithmetic as necessary. This relies on knowing what to look for, and how that is aligned to the start of the partition and to to metaslab addresses and offsets that determine the writes we actually care about. #2. Use dtrace to watch the actual sector addresses being accessed when examining a pool (e.g. zdb -l). Apply arithmetic as necessary. This relies on dtrace clue. As for the arithmetic.. I'm not certain I've seen, for example, a definitive statement of what the alignment offset is between start-of-partition and zfs data blocks, once various preamble header sectors are allowed for. -- Dan.
pgpHvd7ayeFBy.pgp
Description: PGP signature
_______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
