Sun's ZFS, to hear some people tell it, is the last filesystem anyone will ever need. It slices, it dices, and so on. Why, its magical powers are so great, it can give you all the advantages RAID-5 (or -6) without any of the historical drawbacks!
Except that it doesn't. Give you all of the benefits of RAID-[56], that is. In order for RAID-Z to do its thing, each filesystem block has to be divided into stripes independently. Which is great if you're dealing with a large file divided into 128kB blocks. And not so great if you're dealing with a single 5kB file that's been scattered across 10 data disks, one sector to each. Now you have to do I/O operations on all 10 disks to read the file back (and 12 to write it in the first place, assuming the use of double-parity raidz2), whereas with a conventional RAID with a more reasonable stripe size, it would take only one.
And that's the tradeoff. It might not sound like much, but it's a really big deal if you're trying to store mail in the Maildir format (one file per message!), or any other workload with a lot of small files and random access. System administrators dealing with such things may take to talking about “spindles” as an attribute of a disk array, as metonymy for the rate at which it can handle random I/Os. As in, the mail fileserver has been slow lately; looks like it needs more spindles. So you add disks to it, even though you have plenty of space free. Because it's all about how fast you can move the physical heads around.
In this light, what RAID-Z does is takes all of your disks and turns them into one spindle. Burning half the blocks to do mirroring instead doesn't sound so bad now, does it?
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment