[linux-lvm] 2 LVM questions - LVM and 2.4.19, and extent based striping.

GBanschbach at sandata.com GBanschbach at sandata.com
Tue Nov 26 13:15:02 UTC 2002

Dear All,

      I want to say first that I am very impressed with how well LVM is
working.  Although this is our first LINUX LVM install,  it is so close to
the HP-UX version!   I have a couple of questions.   I have probably missed
the obvious,  but will one of you kind souls give a review of this:
Question 1:
I have set up LVM on a SUSE Enterprise Server, after an upgrade to 2.4.18.
All is well.    Now,   I need to know what the best combination of LVM
version 1.0.5  or 1.0.6  and 2.4.19 or higher.   Or can I just build a
2.4.19 kernel, and do the same things I did to get 2.4.18 working - and use
the same version of LVM I have been using all along?    I may have to get
to at least 2.4.19, so I can use the most stable [ most debugged ] LARGE
memory and SMP version of the kernel I can....   REASON:  I have 16GB of
ram, and 2 CPUs.   I want to make good use of all of it.

I also am curious about something else.     I have read one other posting
about this.   LVM on Linux is nearly identical to LVM on HP-UX.     I saw a
posting concerning what amounts to "extent based striping", as opposed to
striping on Linux which seems to be stripes within extents.     We HP-UX
people tend to want to do "extent based stripes."   This is because we can
take 3 disks, and say to LVM  "When there is a requirement for extents in a
logical volume, draw each new extent in a "round robin" fashion ONE at a
time from each disk."     The performance is very nice.   And we like the
control it gives us.    To illustrate:

3 disk model
Logical Ext.                   Phys  Ext.
0000              0000 on disk1
0001                                       0000 on disk2
0002                                       0000 on disk3

0003                                       0001 on disk1
0004                                       0001 on disk2
0005                                       0001 on disk3

In the linux LVM world, we can't seem to do this......      We get stripes
WITHIN extents, but the extents are still visited serially:

The answer in the posting went kind of like this:   assuming  we will write
64k stripes....and 6 extents total......
Logical Ext.                   Phys  Ext.
0000              0000 on disk1
0001                                       0001 on disk1
0002                                       0002 on disk1
0003                                       0003 on disk1
0004                                       0004 on disk1
0005                                       0005 on disk1
1st  64k on Log. Ext Zero which is Physical Zero on disk1.  The 2nd is on
LE 1 on disk 1.  There is still alot of room on Logical Ext. Zero on disk
1,  because we only used 64k out of 4096k.   Stripe number 4 will go on
Logical Ext. Zero, after the very first one we wrote.... and so on, until
you hit a high enough number of writes to fill the 6 extents (?)   The
question which remains to be answered in my mind is .....what happened to
the multiple disks aspect?   Striping ( as I have been taught and
understand ) works BETTER when it is across MORE spindles round robin.
What benefit do you get by striping within a set of serial extents ?
Since the default allocation [  without stripes ] is to take extents
serially from disk1 until there are no more there and then continue on
disk2,  it should not be a huge code change to tell it:  Take the next
serial extent from disk1,   increment to point to the next available disk
in the group,  take the NEXT serial extent from disk2, etc.     Or is the
code organised in such a way that it is prohibitive to try this ???      If
anyone wants to give me some background and coaching.....  I might be able
to take a tentative stab at it......maybe.     I think it would be better
if the people most familiar with it make the change. But I get the feeling
alot of people would like the option.     The performance gain should be
pretty high.    In the HP-UX world, we set up a file with a list of disks,
then tell lvcreate to use this list to do the distribution.

By the way -  I suspect that striping within extents happens in the HP-UX
example,  but I don't really worry about it due to the way I can map the

Thanks very much,


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