hardware for home use large storage

[Jeremy Chadwick]

On Mon, Feb 15, 2010 at 08:57:10AM +0100, Alexander Leidinger wrote:
> Quoting Dan Naumov <dan.naumov at gmail.com> (from Mon, 15 Feb 2010
> 01:10:49 +0200):
> 
> >Get a dock for holding 2 x 2,5" disks in a single 5,25" slot and put
> >it at the top, in the only 5,25" bay of the case. Now add an
> >additional PCI-E SATA controller card, like the often mentioned PCIE
> >SIL3124. Now you have 2 x 2,5" disk slots and 8 x 3,5" disk slots,
> >with 6 native SATA ports on the motherboard and more ports on the
> >controller card. Now get 2 x 80gb Intel SSDs and put them into the
> >dock. Now partition each of them in the following fashion:
> >
> >1: swap: 4-5gb
> >2: freebsd-zfs: ~10-15gb for root filesystem
> >3: freebsd-zfs: rest of the disk: dedicated L2ARC vdev
> 
> If you already have 2 SSDs I suggest to make 4 partitions. The
> additional one for the ZIL (decide yourself what you want to speed
> up "more" and size the L2ARC and ZIL partitions accordingly...).
> This should speed up write operations. The ZIL one should be zfs
> mirrored, because the ZIL is more sensitive to disk failures than
> the L2ARC: zpool add <pool> log mirror <SSD1pX> <SSD2pX>
> 
> >GMirror your SSD swap partitions.
> >Make a ZFS mirror pool out of your SSD root filesystem partitions
> >Build your big ZFS pool however you like out of the mechanical
> >disks you have.
> >Add the 2 x ~60gb partitions as dedicated independant L2ARC devices
> >for your SATA disk ZFS pool.
> 
> BTW, the cheap way of doing something like this is to add a USB
> memory stick as L2ARC:
> http://www.leidinger.net/blog/2010/02/10/making-zfs-faster/
> This will not give you the speed boost of a real SSD attached via
> SATA, but for the price (maybe you even got the memory stick for
> free somewhere) you can not get something better.

I had a feeling someone would bring up L2ARC/cache devices.  This gives
me the opportunity to ask something that's been on my mind for quite
some time now:

Aside from the capacity different (e.g. 40GB vs. 1GB), is there a
benefit to using a dedicated RAM disk (e.g. md(4)) to a pool for
L2ARC/cache?  The ZFS documentation explicitly states that cache
device content is considered volatile.

Example:

# zpool status storage
  pool: storage
 state: ONLINE
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        storage     ONLINE       0     0     0
          mirror    ONLINE       0     0     0
            ad10    ONLINE       0     0     0
            ad14    ONLINE       0     0     0

errors: No known data errors

# mdconfig -a -t malloc -o reserve -s 256m -u 16
# zpool add storage cache md16
# zpool status storage
  pool: storage
 state: ONLINE
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        storage     ONLINE       0     0     0
          mirror    ONLINE       0     0     0
            ad10    ONLINE       0     0     0
            ad14    ONLINE       0     0     0
        cache
          md16      ONLINE       0     0     0


And removal:

# zpool remove storage md16
# mdconfig -d -u 16
#

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |