[OpenIndiana-discuss] SMR disks

[Andrew Gabriel]

On 01/05/2015 07:13, Nick Tan wrote:
> Hi all,
>
> Has anyone tried using SMR disks with ZFS?  I bought a Seagate 8TB SMR disk
> and put it in a esata enclosure for my backups.  I found that zfs send
> would cause the disk to go offline.  My guess is that zfs send is too fast
> and fills the drive write cache.
>
> I tried again with just rsync and this worked fine.

How did you setup the drive?
What filesystem, or just writing to it serially like a tape drive?

SMR disks have some interesting issues with recording, particularly when 
writing non-serially as most filesystems normally do. Since there's no 
SMR support in Illumos, I presume you ran the drive in Drive Managed 
mode - this makes it look like a standard random access drive. However, 
like a flash drive, it will actually be laying the data on the drive out 
very differently from what the host OS/filesystem imagines. Also like a 
flash drive, it will have to do some housekeeping and move blocks of 
data around on the disk and/or re-record large ranges of data previously 
written, so performance from the host system may appear very mixed, 
including some i/o requests which take long enough that with a standard 
magnetic drive you would assume the drive is dying (probably why you saw 
the drive reported as going offline). This may be fine for 
archival/backup data (providing the host system knows to allow a long 
time for i/o), but is less likely to be good for normal filesystem use 
by applications.

There are better ways of driving SMR drives, but they require support in 
the operating system and/or application. One of the uses they are more 
suitable for is a key/value object store, because drives can implement 
the object store layer entirely in the drive firmware hiding the real 
layout from the system, and data is accessed entirely by using keys.

-- 
Andrew