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I'm attempting to connect my desktop (running debian 7) to a QNAP NAS (running QTS 4.1.1) using iSCSI with an MTU=9000. The network infrastructure consists of a cat5e crossover cable between the desktop and the nas.

If I set the MTU on local machine and iSCSI machine to either 1500, 4074, or 7418 everything works fine.

However, if I set MTU to 9000 and issue the same mount command as before then it mounts as a read only filesystem:

mount /dev/sdc1 /mnt/iscsi

After that command is issued, the output of mount -l is:

/dev/sdc1 on /mnt/iscsi type ext4 (rw,relatime,user_xattr,barrier=1,data=ordered)

However as soon as I try to write to the filesystem I receive an error Read-only file system and the output of mount -l becomes:

/dev/sdc1 on /mnt/iscsi type ext4 (ro,relatime,user_xattr,barrier=1,data=ordered)

I've tried mounting with -w and -o rw but neither have any effect. Does anyone have any suggestions on what might be causing this?

I know the nas supports jumbo frames, but could it be that the NIC doesn't support them? Is there any way to check?

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    Do you have a switch between your machines ? Is it supports Jumbo frames ?
    – Dom
    Dec 14, 2014 at 19:14
  • Sorry, I should've mentioned that - I'm using a crossover cable
    – James
    Dec 14, 2014 at 20:02
  • Just don't use jumbo frames...
    – ewwhite
    Dec 14, 2014 at 23:38
  • If at first you don't succeed 'n all?
    – James
    Dec 15, 2014 at 7:30

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