High fragmentation on zpool log

[krad]

Fragmentation isn't really a big issue on SSD's as there are no heads to
move around like on magnetic drives. Also due to wear levelling, you
actually have no idea where a block actually  is on a  memory cell, as the
drive only gives a logical representation of the layout of blocks, not an
actual true mapping.

On 27 November 2015 at 15:24, Kai Gallasch <k at free.de> wrote:

>
> Hi.
>
> Today I had a look at the zpool of a server (FreeBSD 10.2, GENERIC
> kernel, 100d uptime, 96GB RAM) I recently installed.
>
> The pool has eight SAS drives in a raid 10 setup (concatenated mirror
> pairs) and uses a cache and a mirrored log.
>
> The log and cache both are on a pair of Intel SSDs.
>
> # gpart show -l da9
> =>       34  195371501  da9  GPT  (93G)
>          34          6       - free -  (3.0K)
>          40   16777216    1  log-BTTV5234003K100FGN  (8.0G)
>    16777256  178594272    2  cache-BTTV5234003K100FGN  (85G)
>   195371528          7       - free -  (3.5K)
>
>
> Is 85% fragmentation of the log device something to worry about?
>
> Why does zpool list show so unrealistic values for FREE and CAP?
> Is this normal?
>
> Atached: Some output of zpool list.
>
> Regards,
> Kai.
>
>
> (zpool list -v output, ommited columns: EXPANDSZ.,DEDUP,
> HEALTH, ALTROOT)
>
> NAME                             SIZE  ALLOC   FREE  FRAG  CAP
> rpool                           7.25T   440G  6.82T    4%   5%
>   mirror                        1.81T   110G  1.71T    4%   5%
>    gpt/rpool-WMC160D0SVZE          -      -      -      -    -
>    gpt/rpool-WMC160D8MJPD          -      -      -      -    -
>   mirror                        1.81T   110G  1.70T    4%   5%
>    gpt/rpool-WMC160D9DLL2          -      -      -      -    -
>    gpt/rpool-WMC160D23CWA          -      -      -      -    -
>   mirror                        1.81T   110G  1.71T    4%   5%
>    gpt/rpool-WMC160D94930          -      -      -      -    -
>    gpt/rpool-WMC160D9V5LW          -      -      -      -    -
>   mirror                        1.81T   110G  1.71T    4%   5%
>    gpt/rpool-WMC160D9ZV0S          -      -      -      -    -
>    gpt/rpool-WMC160D5HFT6          -      -      -      -    -
>   mirror                        7.94G  43.2M  7.90G   85%   0%
>    gpt/log-BTTV523401U4100FGN      -      -      -      -    -
>    gpt/log-BTTV5234003K100FGN      -      -      -      -    -
> cache                              -      -      -      -    -
>   gpt/cache-BTTV5234003K100FGN  85.2G   142G  16.0E    0%   166%
>   gpt/cache-BTTV523401U4100FGN  85.2G   172G  16.0E    0%   202%
>
>
>
> --
> PGP-KeyID = 0x70654D7C4FB1F588
>
>
>
>