Hello.
Currently I am working on a RAID-1 created using mdadm (version 2.6.2)
with a local disc and a remote disc connected via iSCSI (open iscsi
for the initiator and iscsi enterprise target).
I am testing the performance of the RAID writing data to the array
disc having my target disc on a simulated link of 1 Mbps, I also
defined my slow device as write-mostly and I am using the write-behind
option so I can write with a decent speed without having to receive
the ACK from the slow device.
My test constantly writes a file on the raid for around 20 minutes,
but then the slow device is marked as faulty; I can recover the raid
removing the device and adding it again, but the resynchronization
process takes a long time (obviously because of the slow device).
Here is the message from the kernel when the slow disc fails:
sd 6:0:0:0: [sdb] Result: hostbyte=DID_OK driverbyte=DRIVER_TIMEOUT,SUGGEST_OK
Summarizing, I have two questions about the behavior of Linux md with
slow devices:
1. Is it possible to modify some kind of time-out parameter on the
mdadm tool so the slow device wouldn't be marked as faulty because of
its slow performance.
2. Is it possible to control the "buffer" size of the RAID?, in other
words, can I control the amount of data I can write to the local disc
before I receive an acknowledgment from the slow device when I am
using the write-behind option.
Any help is highly appreciated.
Thanks.
Juan Aristizabal.
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