| 1 |
On Thu, Sep 19, 2013 at 5:37 PM, Tanstaafl <tanstaafl@×××××××××××.org> wrote: |
| 2 |
> On 2013-09-19 3:44 AM, Hinnerk van Bruinehsen <h.v.bruinehsen@fu- |
| 3 |
> |
| 4 |
>> You should definitely determine the right value for ashift on pool |
| 5 |
>> creation |
| 6 |
>> (it controls the alignment on the medium). It's an option that you afaik |
| 7 |
>> can only set |
| 8 |
>> on filesystem creation and therefore needs a restart from scratch if you |
| 9 |
>> get it |
| 10 |
>> wrong. |
| 11 |
>> According to the illumos wiki it's possible to run a mixed pool (if you |
| 12 |
>> have |
| 13 |
>> drives requiring different alignments[1]) |
| 14 |
>> If in doubt: ask ryao (iirc given the right information he can tell you |
| 15 |
>> which |
| 16 |
>> are the right options for you if you can't deduce it yourself). |
| 17 |
>> Choosing the wrong alignment can cause severe performance loss (that's not |
| 18 |
>> a ZFS issue but happened when 4k sector drives appeared and tools like |
| 19 |
>> fdisk |
| 20 |
>> weren't aware of this). |
| 21 |
> |
| 22 |
> |
| 23 |
> Yikes... |
| 24 |
> |
| 25 |
> Ok, shouldn't there be a tool or tools to help with this? Ie, boot up on a |
| 26 |
> bootable tools disk on the system with all drives connected, then let it |
| 27 |
> 'analyze' your system, maybe ask you some questions (ie, how you will be |
| 28 |
> configuring the drives/RAID, etc), then spit out an optimized config for |
| 29 |
> you? |
| 30 |
> |
| 31 |
> It is starting to sound like you need to be a dang engineer just to use |
| 32 |
> ZFS... |
| 33 |
> |
| 34 |
|
| 35 |
Just do ashift=12 and you're good to go. No need to analyze further. |
| 36 |
|
| 37 |
The reason I said that because in the future, *all* drives will have 4 |
| 38 |
KiB sectors. Currently, many drives still have 512 B sectors. But when |
| 39 |
one day your drive dies and you need to replace it, will you be able |
| 40 |
to find a drive with 512 B sectors? |
| 41 |
|
| 42 |
Unlikely. |
| 43 |
|
| 44 |
That's why, even if your drives are currently of the 'classic' 512 B |
| 45 |
ones, go with ashift=12 anyway. |
| 46 |
|
| 47 |
For SSDs, the situation is murkier. Many SSDs 'lie' about their actual |
| 48 |
sector size, reporting to the OS that their sector size is 512 B (or 4 |
| 49 |
KiB). No tool can pierce this veil of smokescreen. The only way is to |
| 50 |
do research on the Internet. |
| 51 |
|
| 52 |
IIRC, a ZFS developer has embedded -- or planned to embed -- a small |
| 53 |
database into the ZFS utilities to conclusively determine what |
| 54 |
settings will be optimal. I forgot who exactly. Maybe @ryao can pipe |
| 55 |
in (hello Richard! If you're watching this thread, feel free to add |
| 56 |
more info). |
| 57 |
|
| 58 |
|
| 59 |
Rgds, |
| 60 |
-- |
| 61 |
FdS Pandu E Poluan |
| 62 |
~ IT Optimizer ~ |
| 63 |
|
| 64 |
• LOPSA Member #15248 |
| 65 |
• Blog : http://pepoluan.tumblr.com |
| 66 |
• Linked-In : http://id.linkedin.com/in/pepoluan |
Archives