Dear all :
I have some(or lots, more precisely) questions when tracing the mirror code!
file : dm-raid.c
function : mirror_map
1.===================================
Dose there any better solution to test the performance of the device-mapper core
and all target type module ?
====================================
2. ===================================
First is the kcopyd necessary in dm-mirror (in previous mail)?
Because dm-mirror will submit each writing bio to all mirror block devices.
It seems kcopyd does not have any works to do in mirror_map().
More precisely :
In log ctr, all region will be logged as non-sync(core-log) or
previous setting(disk-log).
When starting mirror, mirror daemon will recovery these non-sync
region by invoking kcopyd daemon
accroding to the dirty log object. But after these regions are all in
synchronous.... kcopyd has nothing to do... Is that right ?
Here are some problems that confuses me for a log time...
1. The only work of kcopyd is only available in configure time (after
log->ctr, since all in-sync bits are set to 0, whatever core-log or
disk-log) ?
On the other hand, Does kcopyd work nothing after the mirror
configure time ?
1-1. We have to re-sync all regions after re-booting if we use core-log ?
1-2. Even for disk-log...... kcopyd wroks only in configure time
(first time to construct a disk log) ?
2. There seems a lack for dm-mirror interface (dm-raid1.c)
to set/clear the sync state for a region.
Under what other conditions the kcopyd will work ?
Whould we implement as following :
s1. write bio to default mirror device.
s2. set the corresponding region to non-sync state
s3. invoke kcopyd to recovery this region to all mirror devices (set
the region in recovering.)
s4. kcopyd callback function set the region in-sync.
Is this better ? Any issue (like read/write race in default mirror device) ?
In my option, it would make the use of kcopyd more, and determine the
region state
(in-sync, non-sync) more precisely...
But any racing condition occurs (read/write race...etc)?
====================================
In mirror_map function
3. =====================
r = ms->rh.log->type->in_sync(ms->rh.log,
bio_to_region(&ms->rh, bio), 0);
/*
* *********** HERE ***********
* When does the in_sync() return < 1 ?
* It is strange here since in_sync(core_in_sync) return
* either 0(non-sync) or 1(in-sync).
*/
if (r < 0 && r != -EWOULDBLOCK)
return r;
/* Simply return -EIO ? */
if (r == -EWOULDBLOCK) /* FIXME: ugly */
r = DM_MAPIO_SUBMITTED;
/*
* We don't want to fast track a recovery just for a read
* ahead. So we just let it silently fail.
* FIXME: get rid of this.
*/
if (!r && rw == READA)
return -EIO;
4. =============================================
/*
* *********** HERE ***********
* What followed seems duplicate in do_read().
* Why not implemented as follows as simple:
*
* queue_bio(ms, bio, rw);
* return DM_MAPIO_SUBMITTED;
*
* and let daemon to process reading ? Any performance issue ?
*/
if (!r) {
/* Pass this io over to the daemon */
queue_bio(ms, bio, rw);
return DM_MAPIO_SUBMITTED;
}
m = choose_mirror(ms, bio->bi_sector);
if (!m)
return -EIO;
map_bio(ms, m, bio);
return DM_MAPIO_REMAPPED;
===================================================
regard.
By Penguin, Kaohsiung, Taiwan.
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