[linux-lvm] snapshots+highmem+'md raid5'

[Herbert Poetzl]

On Sat, Jun 14, 2003 at 02:45:11PM +0200, Svetoslav Slavtchev wrote:
> Quoting Herbert Poetzl <herbert at 13thfloor.at>:
> 
> > didn't say that this would be bad, only that
> > it live-locked my system at high load ...
> > the tenor on the list was: "highmem is bad"
> > so I turned it off, and, voila the system was
> > stable again ...
> 
> snapshots + md + highmem is very bad
> if you don't use snapshots its perfectly OK

I have to disagree, live-lock under high load is
not perfectly OK, at least not for me ... 8-)

> > another, yet unexplained issue, is that on
> > another system, 4 SCSI discs, form a RAID 5
> > array (software) which gives about 35MB
> > throughput, but lvm 1.0.7 ontop of that md
> > only has about 18MB throughput ...
> 
> I've a bit defferent figures from a onboard HPT374 4 ide chanels with
> 4 IBM deskstars 80Gb, but i still lose 8-15Mb/s compared to plain md :
> 
> /dev/hda:
>  Timing buffer-cache reads:   128 MB in  0.41 seconds =308.43 MB/sec
>  Timing buffered disk reads:  64 MB in  1.42 seconds = 45.07 MB/sec
> [root at svetljo root]# hdparm -tT /dev/md13
> 
> /dev/md13:
>  Timing buffer-cache reads:   128 MB in  0.41 seconds =316.05 MB/sec
>  Timing buffered disk reads:  64 MB in  1.08 seconds = 59.26 MB/sec
> [root at svetljo root]# hdparm -tT /dev/data
> data   data2
> [root at svetljo root]# hdparm -tT /dev/data2/Movies
> 
> /dev/data2/Movies:
>  Timing buffer-cache reads:   128 MB in  0.41 seconds =316.05 MB/sec
>  Timing buffered disk reads:  64 MB in  1.32 seconds = 48.30 MB/sec

I posted my results a month ago, but I digged them
out again, because together with your results, it
sheds new light on that issue ...

mine          	raw disc    md(raid5)   lvm on md
-----------------------------------------------------
hdparm          14694.40    34129.92    18667.52
dd 1024k        14988.22    34732.56    18647.98
dd 32k          15516.06    33945.48    18862.67

yours          	raw disc    md(raid5)   lvm on md
-----------------------------------------------------
hdparm          46151.68    60682.24    49459.20 

in both cases the lvm lies about 3MB over the raw
disc throughput, regardless of the md values ...

maybe this could lead to some explanation of this
issue ... unfortunately there was no reply to my
posting over the last month *sigh*

best,
Herbert

> time dd if=/dev/zero of=/mnt/Movies/BigFile.tm bs=1M count=4000
> 4000+0 records in
> 4000+0 records out
> 0.02user 15.57system 1:37.10elapsed 16%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (148major+40minor)pagefaults 0swaps
> 
> best,
> 
> Svetljo
> 
> 
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