[dm-devel] Unnecessary scanning of all SCSI devices in mpath_persistent_reserve_in/out command, sent to a specific mpath device,

[Satyajit Deshmukh]

Hello,

This is related to an issue raised by Richard Sharpe recently.
https://www.redhat.com/archives/dm-devel/2015-August/msg00154.html

The RPM that was used-
http://vault.centos.org/6.6/updates/Source/SPackages/device-mapper-multipath-0.4.9-80.el6_6.3.src.rpm

*Problem:*
>From reading the source code, I understand that 2 APIs defined in
libmpathpersist/mpath_persist.c do a system-wide scan of all SCSI devices
by sending asynchronous IO (io_submit calls) to test if SCSI device path is
good:



*extern int mpath_persistent_reserve_in (int fd, int rq_servact, struct
prin_resp *resp, int noisy, int verbose);extern int
mpath_persistent_reserve_out ( int fd, int rq_servact, int rq_scope,
unsigned int rq_type, struct prout_param_descriptor *paramp, int noisy,
int verbose);*

So, to get a persistent reservation on a single /dev/mapper/mpathx device
on a system having 400 SCSI devices, current code does 400 reads of 4K
each.(512 bytes of 8 sectors), leading to huge delays.

*Root cause of the system wide device scan:*
The system-wide scan is triggered by setting the DI_CHECKER bit in the
flags argument to a path_discovery() API:










* if (path_discovery(pathvec, conf, DI_SYSFS | DI_CHECKER)) {    ret =
MPATH_PR_DMMP_ERROR;    goto out1;  }  /* get info of all paths from the dm
device */  if (get_mpvec (curmp, pathvec, alias)){    condlog(0, "%s:
failed to get device info.", alias);    ret = MPATH_PR_DMMP_ERROR;    goto
out1;  }  *

I believe the DI_CHECKER bit in path_discovery() API is not needed.
This is because the path status checking is already done on the specific
mpath device in the get_mpvec API.

The get_mpvec() gets the required struct multipath *mpp, and then invokes
pathinfo(pp, conf->hwtable, DI_CHECKER) for each path in the multipath dev.
And this checks state of each path for the multipath device.

* if (mask & DI_CHECKER) {    pp->chkrstate = pp->state = get_state(pp, 0);*

*Possible solution:*
I have a patch ready that simply removes the DI_CHECKER bit from the
path_discovery() calls inside mpath_persistent_reserve_in() and
mpath_persistent_reserve_out(). Attached the patch.
I have tested out the following PRIN and PROUT commands and don't see any
errors.
mpathpersist -i -k /dev/mapper/mpathig
mpathpersist -i -r /dev/mapper/mpathig
mpathpersist --out --register --param-rk=0A041D3d /dev/mapper/mpathig
mpathpersist --out --register --param-sark=0A041D3d /dev/mapper/mpathig
mpathpersist --out --reserve --param-rk=0A041D3D --prout-type=5 --verbose=3
/dev/mapper/mpathig

Request someone to review the patch, suggest changes and guide me in fixing
this issue.

Thanks,
Satyajit
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