Has anyone done a comparison of the reliability and performance of a mirrored zpool vs. a non-redundant zpool using ditto blocks? What about a gut-instinct about which will give better performance? Or do I have to wait until my Thumper arrives to find out for myself? Also, in selecting where a ditto block is written, (other than "far away") does the system take into account the disk''s path, so for example, would it write both copies down a single controller? --Joe
On 5/23/07, Moore, Joe <jmoore at ugs.com> wrote:> Has anyone done a comparison of the reliability and performance of a > mirrored zpool vs. a non-redundant zpool using ditto blocks? What about > a gut-instinct about which will give better performance? Or do I have > to wait until my Thumper arrives to find out for myself? > > Also, in selecting where a ditto block is written, (other than "far > away") does the system take into account the disk''s path, so for > example, would it write both copies down a single controller? >ZFS''s SPA will write ditto blocks to vdevs which are logically adjacent to each other in the pool (if there''s space); in the case where there aren''t other vdevs with remaining space, ditto blocks are written to the same vdev separated by 1/8 of the device''s size. As a result, while it makes data loss much less likely, ditto blocks don''t guarantee redundancy like mirroring or RAID-Z would. And as far as I''m aware if a disk dies, and you lose a single non-redundant vdev from your pool, ZFS will panic and you will be unable to reopen the pool. James
Moore, Joe wrote:> Has anyone done a comparison of the reliability and performance of a > mirrored zpool vs. a non-redundant zpool using ditto blocks? What about > a gut-instinct about which will give better performance? Or do I have > to wait until my Thumper arrives to find out for myself?It all depends on the configuration. For a single disk system, copies should generally be faster than mirroring. For multiple disks, the performance should be similar as copies are spread out over different disks.> Also, in selecting where a ditto block is written, (other than "far > away") does the system take into account the disk''s path, so for > example, would it write both copies down a single controller?Paths are not considered, post creation. The copies are placed on adjacent vdevs (N, N+1) and cycle through the vdevs. So if you created the pool with adjacent vdevs on different controllers, then you''ll get some diversity. Note: as the file system becomes full, the knowledge of the mapping of copies becomes less predictable. For example, if I have a zpool (stripe) with a 200 GByte vdev and 500 GByte vdev, then with copies=2 I will have effectively 350 GBytes of available space, (200+500)/2. As we get beyond 200 GBytes of space used, both copies will be allocated on the 500 GByte disk. With mirrors, I will have only 200 GBytes of available space min(200, 500). In short, redundant copies of data is better than no redundant copies of data. Whether you use copies=2 or mirroring will be a design decision that you must make. Remember, you can always attach or detach mirrors, so you do have some flexibility to change policies later. -- richard
Richard Elling wrote:> It all depends on the configuration. For a single disk system, copies > should generally be faster than mirroring. For multiple disks, the > performance should be similar as copies are spread out over different > disks.Here''s a crazy idea: could we use zfs on dvd for s/w distribution? With ditto blocks, there''d be additional protection against media errors. Henk
On 24-May-07, at 6:26 AM, Henk Langeveld wrote:> Richard Elling wrote: >> It all depends on the configuration. For a single disk system, >> copies >> should generally be faster than mirroring. For multiple disks, the >> performance should be similar as copies are spread out over different >> disks. > > Here''s a crazy idea: could we use zfs on dvd for s/w distribution? > With > ditto blocks, there''d be additional protection against media errors.Definitely. I used to have a rule, always send two copies of any floppy disk... DVDs are no different (unreliable). --Toby> > Henk > > _______________________________________________ > zfs-discuss mailing list > zfs-discuss at opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss