> On Mar 22, 2016, at 7:41 AM, Chris Siebenmann <c...@cs.toronto.edu> wrote: > >>> This implicitly assumes that the only reason to set ashift=12 is >>> if you are currently using one or more drives that require it. I >>> strongly disagree with this view. Since ZFS cannot currently replace >>> a 512n drive with a 512e one, I feel [...] >> >> *In theory* this replacement should work well if the lie works *correctly*. >> In ZoL, for the "-o ashift" is supported in "zpool replace", the >> replacement should also work in mixed sector sizes. >> And in illumos the whitelist will do the same. >> What errors have you ever seen? > > We have seen devices that changed between (claimed) 512n and > (claimed) 512e/4k *within the same model number*; the only thing that > distinguished the two was firmware version (which is not something that > you can match in sd.conf). This came as a complete surprise to us the > first time we needed to replace an old (512n) one of these with a new > (512e) one. > > The sd.conf whitelist also requires a reboot to activate if you need > to add a new entry, as far as I know. > > (Nor do I know what happens if you have some 512n disks and some > 512e disks, both correctly recognized and in different pools, and > now you need to replace a 512n disk with a spare 512e disk so you > change sd.conf to claim that all of the 512e disks are 512n. I'd > like to think that ZFS will carry on as normal, but I'm not sure. > This makes it somewhat dangerous to change sd.conf on a live system.)
What is missing from http://wiki.illumos.org/display/illumos/ZFS+and+Advanced+Format+disks <http://wiki.illumos.org/display/illumos/ZFS+and+Advanced+Format+disks> is: 1. how to change the un_phy_blocksize for any or all uns 2. how to set a default setting for all drives in sd.conf by setting attributes to the "<vid+pid>" of "" (see sd(7d)) I am aware of no new HDDs with 512n, so this problem will go away for HDDs. However, there are many SSDs that work better with un_phy_blocksize = 8192 and some vendors set sd.conf or source appropriately. -- richard > >>> For many usage cases, somewhat more space usage and perhaps >>> somewhat slower pools are vastly preferable to a loss of pool >>> redundancy over time. I feel that OmniOS should at least give you >>> the option here (in a less crude way than simply telling it that >>> absolutely all of your drives are 4k drives, partly because such >>> general lies are problematic in various situations). >> >> The whitelist (sd.conf) should fit into this consideration. But not >> sure how mixed sector sizes impact the performance. > > Oh, 512e disks in a 512n pool will probably have not great performance. > ZFS does a lot of unaligned reads and writes, unlike other filesystems; > if you say your disks are 512n, it really believes you and behaves > accordingly. > > - cks
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