One of the plenary sessions on the first day of the Linux Storage,
Filesystem, and Memory Management (LSFMM) Summit concerned Linux support
for shingled magnetic recording (SMR) devices. These next-generation hard
disks have a number of interesting characteristics that will be challenging
to fully support. Martin Furuhjelm led a discussion among a few drive vendor
representatives and the assembled kernel developers about the latest
developments in SMR-land.
There are three types of SMR drives: device managed, host aware, and host
managed. Device-managed drives will essentially act just like regular disk drives,
though the
translation layer in the drive may cause unexpected performance degradation
at times (much like flash devices today). Existing drivers don't need to
change for device-managed disks. The discussion concentrated mostly on
host-aware drives (where the host should try to follow the
requirements for shingled regions) and host-managed devices (where the
requirements must be followed).
SMR drives will be made up of multiple zones, some that are "normal" and
allow random reads and writes throughout the zone, and some that can only be written
sequentially. For the sequential zones, there is a write pointer
maintained for each zone that corresponds to where the next write must go.
Depending on the mode, writing elsewhere in the zone will either be an
error (in host-managed devices) or will lead to some kind of remapping of
the write (for host-aware devices). That remapping may lead to latency
spikes due to garbage collection at some later time.
Two new SCSI commands have been added, one to query what zones exist on the
drive and another to reset the write pointer to the
beginning of a particular zone. To get the best performance, an SMR-aware driver will
need to only write sequentially to the sequential
zones (that will likely make up most of the disk), but if it fails to do
so, it will be a fatal error only on
host-managed drives. For that reason, most of the kernel developers seemed
to think the first SMR drives are likely to be host-aware since those will
work (though perhaps poorly at times) with today's software.
The T10 technical committee (for SCSI
interface standards) is currently working on finishing the standards for
SMR, so it is important that Linux developers make any concerns they have
with the drafts known soon. Ted Ts'o noted that the drafts are available
from the T10 site (Furuhjelm recommended looking for "ZBC"). In addition,
more information on SMR and Linux can be found in a writeup from last year's LSFMM.
There were some questions about the zone reporting functionality, but much
of that is still up in the air at this point. Currently, all zones are
expected to be the same size, though there is a belief that will change
before the draft is finalized. There has also been talk of adding a
filtering capability on the query, so that only zones fitting a particular
category (active, full, sequential-only, etc.) would be returned.
The overall sense was that kernel developers are waiting for hardware
before trying to determine how best to support SMR in Linux. No major
complaints about the draft interface were heard, but until hardware hits, it
will be difficult for anyone to determine where the problems lie.
[ Thanks to the Linux Foundation for travel support to attend LSFMM. ]
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