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Old 09-09-2011, 07:34 PM   #1
needsleep
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CentOS 5.6: LVM, LV, PV, external SCSI, USB - discussion and questions


First I will need to discuss a little bit, followed by my questions.

I'm trying to understand LVM as it relates to mounting and un-mounting of filesystems and to learn what actions are needed (if any) to attach and remove an external drive that has been placed into the LVM system.

I have two LVM Physical Volumes (PV) on my system. The first is comprised of my 2 internal IDE drives and provides space for logical volumes (LV) which map to /,/boot,/opt and /orafs. These LV's are mounted.

I have a SCSI external drive attached to my computer. It's 4 partitions have been allocated to a single, second PV. This PV has yet to have any LV's created from it, so, naturally, I have nothing mounted for this drive.

In Windows XP, to "un-mount" an external (USB) device requires me to run the "Safely Remove Hardware" widget, which dumps any write buffers that are outstanding, before the device is powered down.

(Q1) Is CentOS also capable of creating a similar possibility - having buffer(s) that need to get dumped before device removal? Let us consider all questions from both USB connections and SCSI connections.

(Q2) If Q1 is yes, is there an analagous program to run for an external drive connected to CentOS?

(Q3) Is the act of un-mounting (umount) equivalent to dumping outstanding buffers to the device?

(Q4) What if I just turned off my SCSI drive? Will that have a disastrous effect on the PV that it is mapped to? I expect to use this drive for backups.

(Q5) Since this is the first of many external backup drives, does it make sense to even place them within the context of LVM?

(Q6) If I created a PV out of two external drives, would both have to be online at the same time for me to be able to access one of them?

Thanks,
 
Old 09-11-2011, 07:17 PM   #2
chrism01
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You may want these for ref
http://tldp.org/HOWTO/LVM-HOWTO/
http://sunoano.name/ws/public_xhtml/lvm.html

You check if any process has open files on the LVM, look at fuser http://linux.die.net/man/1/fuser & lsof http://linux.die.net/man/8/lsof
 
Old 09-11-2011, 10:10 PM   #3
smallpond
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Quote:
Originally Posted by needsleep View Post
(Q1) Is CentOS also capable of creating a similar possibility - having buffer(s) that need to get dumped before device removal? Let us consider all questions from both USB connections and SCSI connections.
Yes. Filesystems use cache, which is flushed in the background.
Quote:
(Q2) If Q1 is yes, is there an analagous program to run for an external drive connected to CentOS?
Not quite. umount writes the data needed to properly close the filesystem, then sync schedules it to be written to disk, but neither one waits for the writes to complete. eject dooes this for removable devices like USB flash drives, but not for disk
Quote:
(Q3) Is the act of un-mounting (umount) equivalent to dumping outstanding buffers to the device?
Nope
Quote:
(Q4) What if I just turned off my SCSI drive? Will that have a disastrous effect on the PV that it is mapped to? I expect to use this drive for backups.
Don't do that
Quote:
(Q5) Since this is the first of many external backup drives, does it make sense to even place them within the context of LVM?
LVM doesn't matter. The above applies to filesystems on non-LVM disk also.
Quote:
(Q6) If I created a PV out of two external drives, would both have to be online at the same time for me to be able to access one of them?
If you want redundancy, you want an md (RAID).
 
  


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