[ENBD] RAID1 performance

[Ulrich Hahn]

The last few days I did some tests with the new RAID1 code.

"make test" was flawless, but changing to 2 servers exporting
100 GB each revealed some problems.

At first it seemed to work not at all (in fact it did work, but it was
awfully slow (>2 hours to build the filesystem)). On the server side I
got a lot of messages like:

nbd-shmem 15162<#  53>: maybe_update_seqno_ondisk server writes req seqno
8544 (ondisk +2)
enbd-server 15159<# 994>: do_srv_write server (0) loses patience waiting
with req 8659, sector 176520-176527

these vanished when I changed from "-w 10000" to "-w 0" on the servers
(this seemed reasonable to me).

Now I was able to make a fs and do some simple tests (dd with 4k
blocksize).
With a single nbd device it took 1:40 to dd 2 GB of data, that's
20 MB/s. Using the RAID1 setup the same transfer took 8:30, that's
4 MB/s. Monitoring traffic on the NIC the single device showed
oszillations between 15 and 30 MB/s, while the NICs in RAID setup
oszillated between 0 MB/s and 12 MB/s.

Any comments on that?

Uli


: Dr. Ulrich Hahn             email: ulrich.hahn@zdv.uni-tuebingen.de :
: Zentrum f=FCr Datenverarbeitung            Tel: + (49) 07071 29 70315 :
: Universit=E4t T=FCbingen                     FAX: + (49) 07071 29  5912 :
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