On 07/09/2015 08:41 AM, Doug Dumitru wrote:> Mr. Hellwig,
On Wed, Jul 8, 2015 at 12:38 PM, Christoph Hellwig <hch@xxxxxxxxxxxxx
<mailto:hch@xxxxxxxxxxxxx>> wrote:
On Wed, Jul 08, 2015 at 09:21:21AM -0700, Doug Dumitru wrote:
> I have a "smart" block device that can implement multi-segment atomic
> writes.
How about submitting your driver upstream first and then we can work
with you on an API that fits the devices and the consumers needs.
I usually like to start with an interface and then implement the
driver's from there.
In this case, this is a block-level interface that supports new
functionality (atomic writes). In the past, you would approach this
type of problem by having the atomic and user layers as a monolithic
solution. Consider database updates and the complexity that they go
through to insure database integrity. If a block device could provide a
database with an atomic update interface, the database would get a lot
simpler. The same discussion holds true for file systems. Depending on
the atomic update implementation, you might end up in the same place in
terms of total code, but you might also end up somewhere completely
different.
The impetus for this is some research on file system "write
amplification". In general, file system design seems to be heading in
the direction of higher and higher write amplification. For example,
the tree structure of zfs is shockingly inefficient in terms of write
overhead. This is happening at the same time as Flash is becoming
popular but is also moving to smaller and smaller geometries. So write
efficiency is becoming more and more important.
By decoupling the atomic update semantics from file system and other
block device "users", this gives devices the opportunity to implement
atomic updates internal to or in cooperation with Flash management
algorithms. In theory, you can implement atomic updates without any
extra writes. In practice, some devices will be better than others.
I was hoping to stumble across someone interested in this as a concept,
or someone who has researched this area, as I don't have any near
production existing code. I could pretty easily hack in a couple of
extra fields in struct bio that would accomplish what I see, but others
might have differing input.
Hello Doug,
When designing such an API, please try to stay close to the semantics of
the already standardized SCSI commands. As you probably know the Linux
SCSI core has been implemented as a block driver. Any new command that
is added to the Linux block layer has to be translated by the Linux SCSI
core into a SCSI command. An example of a patch series that adds support
for a new block layer primitive is the patch series that adds
compare-and-write support
(http://thread.gmane.org/gmane.linux.scsi/95869). Although that patch
series is not yet upstream I think it is a good example of how to add
new functionality to the block layer and SCSI core.
Bart.
--
dm-devel mailing list
dm-devel@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/dm-devel
