Hi,
I have some questions regarding the use of logical volumes with FC3 systems:
1. What happens if a LV spans over mutliple hard disks and one of the disks fails while the server is running? Can the server still keep on running on the other disks as long as no data from the broken disk is needed? Or does everything just crash once one of the disks dies? As an extreme case, lets assume the broken disk does not contain any files or directories that are used at all.
2. How big is the performance loss? Obviously, an additional layer in the file system hierarchy does mean additional mapping and handling, meaning additional processor usage, meaning performance loss. In most cases this probably does not matter, but what if I use the server for example as a mysql database server?
Thanks,
MARK
Mark wrote:
Hi,
I have some questions regarding the use of logical volumes with FC3 systems:
- What happens if a LV spans over mutliple hard disks and one of the disks fails while the server is running?
Can the server still keep on running on the other disks as long as no data from the broken disk is needed? Or does everything just crash once one of the disks dies? As an extreme case, lets assume the broken disk does not contain any files or directories that are used at all.
Others may know more since the only LV stuff I do is where the PVs are actually RAID1, RAID5 or RAID10 disk arrays and I can survive the loss of a drive.
In your example you aren't striping, so the volume should remain available as long as the volume doesn't extend onto the failed drive. If you had striped the LV across the drives, then a failed drive will fail the LV.
- How big is the performance loss? Obviously, an additional layer in the file system hierarchy does mean additional mapping and
handling, meaning additional processor usage, meaning performance loss. In most cases this probably does not matter, but what if I use the server for example as a mysql database server?
There should be no impact as far as performance is concerned since you're only involving one spindle at a time. I sure-as-hell wouldn't trust the volume for long--you're on very thin ice. ---------------------------------------------------------------------- - Rick Stevens, Senior Systems Engineer rstevens@vitalstream.com - - VitalStream, Inc. http://www.vitalstream.com - - - - Vegetarian: Old Indian word for "lousy hunter" - ----------------------------------------------------------------------
- How big is the performance loss? Obviously, an
additional layer in
the file system hierarchy does mean additional mapping and
handling,
meaning additional processor usage, meaning performance
loss. In most
cases this probably does not matter, but what if I use the
server for
example as a mysql database server?
There should be no impact as far as performance is concerned since you're only involving one spindle at a time. I sure-as-hell wouldn't trust the volume for long--you're on very thin ice.
What part would you not trust exactly? You mean the LVM software itself? Or the hardware underneath it (because it's not RAIDed)?
Thanks,
MARK
On Wednesday, Mar 30, 2005, at 19:10 US/Central, Mark wrote:
- What happens if a LV spans over mutliple hard disks and one of the
disks fails while the server is running? Can the server still keep on running on the other disks as long as no data from the broken disk is needed? Or does everything just crash once one of the disks dies? As an extreme case, lets assume the broken disk does not contain any files or directories that are used at all.
Short answer: you're totally hosed. Long answer: you're somewhere between partly hosed and totally hosed. But even if you're partly hosed, by the time you fix things and get back up to speed you will have wished you had setup a RAID.
- How big is the performance loss? Obviously, an additional layer in
the file system hierarchy does mean additional mapping and handling, meaning additional processor usage, meaning performance loss. In most cases this probably does not matter, but what if I use the server for example as a mysql database server?
You're running a server. You want data redundancy. You want performance. Sounds like you want at a minimum RAID-5 and possibly RAID-10. LVM gives you none of the above. The only thing LVM gives you is flexibility in how you manage your disk space, the ease with which you can create/reduce/enlarge/remove virtual partitions, i.e. logical volumes.
But the best way to answer your questions is to actually setup a test system and experiment with it. You don't even need a system with multiple drives. Just partition a single disk with a few (at least 3) extra partitions of say 200 MB each. Then create the LVM and later remove one of the partitions. What do you think will happen? What actually does happen? Now repeat, testing for performance.
And please, let the list know how things go.
Regards, - Robert http://www.cwelug.org/downloads Help others get OpenSource software. Distribute FLOSS for Windows, Linux, *BSD, and MacOS X with BitTorrent