zfs l2arc warmup

[Dmitry Morozovsky]

On Fri, 28 Mar 2014, Joar Jegleim wrote:

[snip most of]

> > Have you measured to see if, or do you otherwise know for sure, that you
> > really do need a ZIL? I suggest not adding a ZIL unless you are certain
> > you need it.
> Yes, I only recently realized that too, and I'm really not sure if a
> zil is required.
> Some small portion of files (som hundre MB's) are served over nfs from
> the same server, if I understand it right a zil will help for nfs
> stuff (?) , but I'm not sure if it's any gain of having a zil today.
> On the other hand, a zil doesn't have to be big, I can simply buy a
> 128GB ssd which are cheap today .

Please don't forget that, unlike L2ARC, if you lost ZIL during sync write, 
you're effectively lost the pool.

Hence, you have two points:

- have ZIL on a enterprise-grade SLC SSD (aircraft-grade prices ;P)
- allocate mirrored ZIL from fraction (rule of thumb if I'm not mistaken was 
"get all of write performance of your low-level disks per 1 second, double it, 
and it will be size of your ZIL) of your existing otherwise used for L2ARC SSDs

We (by all means not at your read pressure) used the second approach, like the 
following:

  pool: br
 state: ONLINE
  scan: resilvered 13.0G in 0h3m with 0 errors on Sun Aug 18 19:52:20 2013
config:

        NAME             STATE     READ WRITE CKSUM
        br               ONLINE       0     0     0
          mirror-0       ONLINE       0     0     0
            gpt/br0      ONLINE       0     0     0
            gpt/br4      ONLINE       0     0     0
          mirror-1       ONLINE       0     0     0
            gpt/br1      ONLINE       0     0     0
            gpt/br5      ONLINE       0     0     0
          mirror-2       ONLINE       0     0     0
            gpt/br2      ONLINE       0     0     0
            gpt/br6      ONLINE       0     0     0
          mirror-3       ONLINE       0     0     0
            gpt/br3      ONLINE       0     0     0
            gpt/br7      ONLINE       0     0     0
        logs
          mirror-4       ONLINE       0     0     0
            gpt/br-zil0  ONLINE       0     0     0
            gpt/br-zil1  ONLINE       0     0     0
        cache
          gpt/br-l2arc0  ONLINE       0     0     0
          gpt/br-l2arc1  ONLINE       0     0     0

where logs/cache are like

root at briareus:~# gpart show -l da9
=>       34  234441581  da9  GPT  (111G)
         34       2014       - free -  (1M)
       2048   16777216    1  br-zil0  (8.0G)
   16779264  217661440    2  br-l2arc0  (103G)
  234440704        911       - free -  (455k)

(this is our main PostgreSQL server, with 8 SASes and 2*Intel3500 SSDs)



-- 
Sincerely,
D.Marck                                     [DM5020, MCK-RIPE, DM3-RIPN]
[ FreeBSD committer:                                 marck at FreeBSD.org ]
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*** Dmitry Morozovsky --- D.Marck --- Wild Woozle --- marck at rinet.ru ***
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