Hi James
On 22/02/13 10:50, James Bottomley wrote:
On Fri, 2013-02-22 at 02:11 +0000, Seymour, Shane M wrote:
First forgive me for using outlook for this, if there are any issues with what I sent let me know and I'll send it again from gmail. This is also my first attempt at a kernel patch so please be gentle.
This patch was written to enable tape statistics via sysfs for the dt
driver based on kernel 3.8.0-rc6. It creates two new files in sysfs
and is based on work done previously in 2005 by Kai Mäkisara. Any
feedback would be greatly appreciated.
I'm afraid we can't do it the way you're proposing. files in sysfs must
conform to the one value per file rule (so we avoid the ABI nastiness
that plagues /proc). You can create a stat directory with a bunch of
files, but not a single file that gives all values.
That was initially my understanding, but when I looked at this the precedent had already been set, see for example the block subsys:
[root@darwin ~]# grep . /sys/block/sd*/stat
/sys/block/sda/stat: 27351 6890 609272 228129 36810 920727 7660304 1333950 0 556889 1562009
/sys/block/sdb/stat: 2369 6762 18890 39003 0 0 0 0 0 4059 39002
[root@darwin ~]#
I can only assume it was implemented this way for the sake of efficiency, eg avoid a huge amount of file open/read/close calls in sar/iostat. It's not unusual for us to see a thousand block devices on enterprise servers, multiply that by the number of above entries and you would be talking about 11 x block-dev-count per iostat read iteration. Okay for tapes we typically don't see anything like this number but the patch just follows the precedent set with the block device. Nevertheless, if opinion is really strong against this we could split each stat out in to a single file.
Assuming sysfs is mounted at /sys the first file
is /sys/bus/scsi/drivers/st/drives which gives a single number
indicating what the largest tape drive instance assigned by st_probe
in the st module is. If it's 4 it possible that st0, st1, st2, and st3
exist on the system. Since tape drives can later be disconnected they
don't have to exist, the count is a hint so it's possible to gather
statistics in a loop with an upper bound. This makes it easier in
iostat to gather statistcs.
I don't really get why you need this. What's wrong with getting the
exact information from /sys/class/scsi_tape?
Again efficiency. The sys/class/scsi_tape is a per device entry whilst /sys/bus/scsi/drivers/st is essentially a global, it seems to make more sense to place a number-of-attached devices here than on a repeated per device basis, especially if for example st0 later disappears, perfectly possible with SAN based devices. If we don't have some hint as to the number of devices currently attached then you don't have any clue as to the device iteration range.
Darren
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