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Full Discussion: Cannot extend logical volume
Operating Systems AIX Cannot extend logical volume Post 302986498 by Scrutinizer on Saturday 26th of November 2016 03:09:17 AM
Old 11-26-2016
Indeed as a rule the combination of lvm striping with high end SAN storage (double or triple striping) is to be avoided, both for reasons of simplicity and performance.

Remarkably perhaps, I have come across situations, where lvm striping actually did make a serious performance difference (improvement) with high end SAN Storage and sequential read IO, but only with a narrow width (say 4-8) at small stripe sizes (128KiB-256KiB).

This was because there was a front-end bottle neck and at the same it was difficult to increase IO queue sizes, which is extra important because of the serial nature of Fibre Channel SANs (the bottle neck could even be observed when the cache-hit ratio was at 100% at the front-end storage level).

By using narrow striping it was possible to increase the effective IO queue size, while at the same time not confusing the prefetch algorithms of the SAN storage. The LUN's in the LVM narrow stripe had to be from different physical disk sets in the backend SAN Storage level, in case of a storage array architecture where this would make a difference). At the SAN storage, this translated in a nice even spread of backend usage, without hot spots.

Situations where this mattered was with databases with quite a bit of sequential read IO. This happened with Oracle databases that were never fully optimized, because the standard query specifications kept changing, which in my experience is the situation that occurs often. Another situation is when out of necessity reports or other batches need to run during on-line usage.

Conversely, I have come across a situation where a large stripe size was used (4 MiB) with a large stripe width (16) and that really confused storage, thwarting the prefetch algorithms, and all IO was done with small sizes, bringing sequential read IO to a crawl, while the storage processors were working overtime.

So as usual in performance tuning: "it depends...." Smilie

Last edited by Scrutinizer; 11-26-2016 at 08:19 AM..
 

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LVEXTEND(8)						      System Manager's Manual						       LVEXTEND(8)

NAME
lvextend - extend the size of a logical volume SYNOPSIS
lvextend [--alloc AllocationPolicy] [-A|--autobackup y|n] [-d|--debug] [-h|-?|--help] [--noudevsync] [-i|--stripes Stripes [-I|--stripesize StripeSize]] {-l|--extents [+]LogicalExtentsNumber[%{VG|LV|PVS|FREE|ORIGIN}] | -L|--size [+]LogicalVolumeSize[bBsSkKmMgGtTpPeE]} [-f|--force] [-n|--nofsck] [-r|--resizefs] [-t|--test] [-v|--verbose] LogicalVolumePath [PhysicalVolumePath[:PE[-PE]]...] DESCRIPTION
lvextend allows you to extend the size of a logical volume. Extension of snapshot logical volumes (see lvcreate(8) for information to cre- ate snapshots) is supported as well. But to change the number of copies in a mirrored logical volume use lvconvert(8). OPTIONS
See lvm for common options. --noudevsync Disable udev synchronisation. The process will not wait for notification from udev. It will continue irrespective of any possible udev processing in the background. You should only use this if udev is not running or has rules that ignore the devices LVM2 cre- ates. -l, --extents [+]LogicalExtentsNumber[%{VG|LV|PVS|FREE|ORIGIN}] Extend or set the logical volume size in units of logical extents. With the + sign the value is added to the actual size of the logical volume and without it, the value is taken as an absolute one. The number can also be expressed as a percentage of the total space in the Volume Group with the suffix %VG, relative to the existing size of the Logical Volume with the suffix %LV, of the remaining free space for the specified PhysicalVolume(s) with the suffix %PVS, as a percentage of the remaining free space in the Volume Group with the suffix %FREE, or (for a snapshot) as a percentage of the total space in the Origin Logical Volume with the suffix %ORIGIN. The resulting value is rounded upward. -L, --size [+]LogicalVolumeSize[bBsSkKmMgGtTpPeE] Extend or set the logical volume size in units of megabytes. A size suffix of M for megabytes, G for gigabytes, T for terabytes, P for petabytes or E for exabytes is optional. With the + sign the value is added to the actual size of the logical volume and with- out it, the value is taken as an absolute one. -i, --stripes Stripes Gives the number of stripes for the extension. Not applicable to LVs using the original metadata LVM format, which must use a sin- gle value throughout. -I, --stripesize StripeSize Gives the number of kilobytes for the granularity of the stripes. Not applicable to LVs using the original metadata LVM format, which must use a single value throughout. StripeSize must be 2^n (n = 2 to 9) -f, --force Proceed with size extension without prompting. -n, --nofsck Do not perform fsck before extending filesystem when filesystem requires it. You may need to use --force to proceed with this option. -r, --resizefs Resize underlying filesystem together with the logical volume using fsadm(8). Examples "lvextend -L +54 /dev/vg01/lvol10 /dev/sdk3" tries to extend the size of that logical volume by 54MB on physical volume /dev/sdk3. This is only possible if /dev/sdk3 is a member of volume group vg01 and there are enough free physical extents in it. "lvextend /dev/vg01/lvol01 /dev/sdk3" tries to extend the size of that logical volume by the amount of free space on physical volume /dev/sdk3. This is equivalent to specifying "-l +100%PVS" on the command line. "lvextend -L+16M vg01/lvol01 /dev/sda:8-9 /dev/sdb:8-9" tries to extend a logical volume "vg01/lvol01" by 16MB using physical extents /dev/sda:8-9 and /dev/sdb:8-9 for allocation of extents. SEE ALSO
fsadm(8), lvm(8), lvcreate(8), lvconvert(8), lvreduce(8), lvresize(8), lvchange(8) Sistina Software UK LVM TOOLS 2.02.95(2) (2012-03-06) LVEXTEND(8)
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