Re: [linux-lvm] Discussion: performance issue on event activation mode

2021-06-15 Thread Zdenek Kabelac 

Dne 15. 06. 21 v 19:03 David Teigland napsal(a):

On Tue, Jun 08, 2021 at 10:39:37AM -0500, David Teigland wrote:

I think it would be an improvement to:

. Make obtain_device_list_from_udev only control how we get the device
   list. Then we can easily default to 0 and readdir /dev if it's better.

. Use both native md/mpath detection *and* udev info when it's readily
   available (don't wait for it), instead of limiting ourselves to one
   source of info.  If either source indicates an md/mpath component,
   then we consider it true.

The second point means we are free to change obtain_device_list_from_udev
as we wish, without affecting md/mpath detection.  It may also improve
md/mpath detection overall.


Here are the initial patches I'm testing (libmpathvalid not yet added)
https://sourceware.org/git/?p=lvm2.git;a=shortlog;h=refs/heads/dev-dct-device-info-1

devices: rework native and udev device info

. Make the obtain_device_list_from_udev setting
   affect only the choice of readdir /dev vs libudev
   for a device name listing.  The setting no longer
   controls if udev is used for device type checks.




While in the local testing it may appear devices on laptops are always fast,
in some cases it may actually be more expensive to check physical
device instead on checking content of udev DB.

So for some users this may result in performance regression as
udevDB is in ramdisk - while there are device where it's opening
may take seconds depending on operating status (disk suspend,
disk firmware upgrade)

(one of lvmetad aspects should have been to avoid waking suspended device)


. Change obtain_device_list_from_udev default to 0.
   A list of device names is obtained from readdir /dev
   by default, which is faster than libudev (especially
   with many devices.)


So we need at least backward compatible setting for those users
where the performance impact would cause regression.


Zdenek

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Re: [linux-lvm] Discussion: performance issue on event activation mode

2021-06-15 Thread David Teigland 
On Tue, Jun 08, 2021 at 10:39:37AM -0500, David Teigland wrote:
> I think it would be an improvement to:
> 
> . Make obtain_device_list_from_udev only control how we get the device
>   list. Then we can easily default to 0 and readdir /dev if it's better.
> 
> . Use both native md/mpath detection *and* udev info when it's readily
>   available (don't wait for it), instead of limiting ourselves to one
>   source of info.  If either source indicates an md/mpath component,
>   then we consider it true.
> 
> The second point means we are free to change obtain_device_list_from_udev
> as we wish, without affecting md/mpath detection.  It may also improve
> md/mpath detection overall.

Here are the initial patches I'm testing (libmpathvalid not yet added)
https://sourceware.org/git/?p=lvm2.git;a=shortlog;h=refs/heads/dev-dct-device-info-1

devices: rework native and udev device info

. Make the obtain_device_list_from_udev setting
  affect only the choice of readdir /dev vs libudev
  for a device name listing.  The setting no longer
  controls if udev is used for device type checks.

. Change obtain_device_list_from_udev default to 0.
  A list of device names is obtained from readdir /dev
  by default, which is faster than libudev (especially
  with many devices.)

. Change external_device_info_source="none" behavior:
  remove libudev device info queries for "none", so
  libudev usage will be avoided entirely. "none"
  remains the default setting.

. Change external_device_info_source="udev" behavior:
  information from libudev is added to the native
  device info rather than replacing the native device
  info. This may be useful if there is some gap in
  the lvm native device info.

. Remove sleep/retry loop when attempting libudev
  queries for device info.  udev info will simply
  be skipped if it's not immediately available.
  Because udev info is supplementary to native
  info, it's not essential to get it.

filter-usable: remove udev dev size check

For the pv_min_size check, always use dev_get_size()
which is commonly used elsewhere, and don't bother
asking libudev for the device size when
external_device_info_source=udev.

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