On Thu, Sep 19, 2013 at 5:37 PM, Tanstaafl <tansta...@libertytrek.org> wrote: > On 2013-09-19 3:44 AM, Hinnerk van Bruinehsen <h.v.bruinehsen@fu- > >> You should definitely determine the right value for ashift on pool >> creation >> (it controls the alignment on the medium). It's an option that you afaik >> can only set >> on filesystem creation and therefore needs a restart from scratch if you >> get it >> wrong. >> According to the illumos wiki it's possible to run a mixed pool (if you >> have >> drives requiring different alignments[1]) >> If in doubt: ask ryao (iirc given the right information he can tell you >> which >> are the right options for you if you can't deduce it yourself). >> Choosing the wrong alignment can cause severe performance loss (that's not >> a ZFS issue but happened when 4k sector drives appeared and tools like >> fdisk >> weren't aware of this). > > > Yikes... > > Ok, shouldn't there be a tool or tools to help with this? Ie, boot up on a > bootable tools disk on the system with all drives connected, then let it > 'analyze' your system, maybe ask you some questions (ie, how you will be > configuring the drives/RAID, etc), then spit out an optimized config for > you? > > It is starting to sound like you need to be a dang engineer just to use > ZFS... >
Just do ashift=12 and you're good to go. No need to analyze further. The reason I said that because in the future, *all* drives will have 4 KiB sectors. Currently, many drives still have 512 B sectors. But when one day your drive dies and you need to replace it, will you be able to find a drive with 512 B sectors? Unlikely. That's why, even if your drives are currently of the 'classic' 512 B ones, go with ashift=12 anyway. For SSDs, the situation is murkier. Many SSDs 'lie' about their actual sector size, reporting to the OS that their sector size is 512 B (or 4 KiB). No tool can pierce this veil of smokescreen. The only way is to do research on the Internet. IIRC, a ZFS developer has embedded -- or planned to embed -- a small database into the ZFS utilities to conclusively determine what settings will be optimal. I forgot who exactly. Maybe @ryao can pipe in (hello Richard! If you're watching this thread, feel free to add more info). Rgds, -- FdS Pandu E Poluan ~ IT Optimizer ~ • LOPSA Member #15248 • Blog : http://pepoluan.tumblr.com • Linked-In : http://id.linkedin.com/in/pepoluan
