Re: [RFC] libata support for async scanning

[Jeff Garzik <jeff@garzik.org>]

Matthew Wilcox wrote:
> On Sat, May 13, 2006 at 01:16:46AM -0400, Jeff Garzik wrote:
>> Matthew Wilcox wrote:
>>> I don't think SATA target scanning occupies a huge amount of boot time,
>>> however it will currently force all async scanners to complete before it
>>> scans.  The Rolls-Royce solution would be somethinkg akin to the new
>>> scsi_scan_host(), but I don't think that's necessaary.  This patch just
>>> acknowledges that async scanning exists and will permit the drive
>>> additions to finish some time after libata has triggered the probe.
>> This seems to add needless complexity.
> 
> It doesn't, honest.
> 
>> The internal probe is complete before the loop begins.  Thus, 
>> ata_scan_scan_host() is simply walking data structures in memory (due to 
>> SCSI simulator) for ATA devices, and for ATAPI devices parallelism is 
>> largely inadvisable :)  Its overkill, and in some rare cases could make 
>> debugging more annoying.
> 
> It won't be parallelised with respect to itself, just with respect to
> other scsi scans.  See my message
> http://marc.theaimsgroup.com/?l=linux-scsi&m=114735805518594&w=2
> 
> Without this patch (on top of that one), libata's calls to
> scsi_scan_target() will block until all asynchronous scans have
> completed (see the conditional call to scsi_complete_async_scans() in
> scsi_scan_target()).  With this patch, libata will scan the targets
> in parallel with other scsi hosts scanning their targets.  It will still
> block, but it'll do it later, when it calls scsi_finish_async_scan().
> 
> Hmm.  Maybe I can improve the API such that scsi_finish_async_scan()
> won't make it block ...
> 
> Anyway, the point here is actually to make your life easier.  If someone
> has a lot of scsi hosts and libata doesn't have this patch, all of a
> sudden people are going to be asking you why libata's taking so long to
> initialise.  And if you debug it, you'll find it's sleeping in
> scsi_complete_async_scans().

First of all, let's establish the large I-dont-care factor on my part. 
I think its an academic exercise that none will notice, with respect to 
libata, but I won't NAK your patch...

	Jeff