Hardware Compression

[Amit Freeman]

I actually posted this at the end of my previous issue's thread... so Im posting it a seperate issue for more readability.

I purchased a SUN C5683A to back up a failing RAID array. With the DIP switch settings to ON ON OFF ON ON ON ON ON, and using "mt -f /dev/nst0 defcompression -1" to disable compression, I have backed up largely uncompressable data onto DDS-4 tape cartridges. However on my last tape, I could only fit 17.3 GB on... the same amount as if HW compression was on. So I ran the LTT tool to check compression, and it wasn't reporting it off as it used to. So, I changed the DIP switches to OFF ON OFF ON ON ON ON ON (compression disabled at poweron with host control), and ran the LTT tool. This time the LTT tool reported that compression is diabled and it is not altering my configuration. However the test STILL gave a compresison ratio of 2.22:1. Even though compression was disabled. So I changed the DIP switches again to be OFF OFF OFF ON ON ON ON ON (compression disabled at poweron with no host control), and ran the LTT tool, which stated that compression was disabled, and wouldn't go through with the test. So I downloaded the latest firmware, updated it using the LTT tool, and tried again, with the same results.

Why did host control of compression suddenly stop working? Am I doing something wrong? Typically I back up as follows:

mt -f /dev/nst0 rewind
mt -f /dev/nst0 setblk 0
mt -f /dev/nst0 defcompression -1
tar -b 128 -T filelist -cvf /dev/nst0

and this has been working on my uncompressible data sets to fit exactly the 20GB native capacity. Why suddenly the change.... overnight?

Attached are all my LTT logs in tar/gzip format.

[CA843380]

Hi,

I don't know why "your host control" of compression stopped working, maybe some Linux/Unix experts can provide some guidance there.

Did you repeat the backup with switch settings OFF OFF OFF ON ON ON ON ON (compression disabled at poweron with no host control), this should give you a garanteed "no compression" operation that you desire.It is obviously not very elegant as it means you will have to alter switch settings if you ever want to run in compression mode in the future and the switches are not exactly made for easy access :)

Regarding LTT, in the first case, with (compression disabled at poweron with host control), LTT will switch on compression to perform the test as requested since host control is permitted and hence that is why you get the test delivered a compression ratio. In the 2nd case, LTT cannot enable compression and so the cannot complete the test.

Cheers,

Dave Dewar

[Amit Freeman]

Yes I did repeat the backup with the compression off and no host control, and the backup proceded as desired (with no compression).

Regarding LTT, when I disabled it during my initial backups, it gave a message saying that compression was disabled, but it will not alter the configuration, and the LTT test proceded with a compression ratio of 1:1. It is as of late that its not. In fact it STILL gives the message taht compression is disabled, and that it is not altering the configuration.... but it still gets the compression ratio of >1:1 (2.22:1)

[CA843380]

Hi,

Ok, I tested this myself

switches OFF OFF OFF ON ON ON ON ON (compression disabled at poweron with no host control)
support ticket reports compression : not present
compression test informs you that drive compression is disabled and test fails with mode select error since it cannot alter the compression ratio due to host control being disabled.

switches to OFF ON OFF ON ON ON ON ON (compression disabled at poweron with host control)
support ticket reports compression : disabled both before and after running the LTT compression test
compression test informs you that compression is disabled but runs the test for you. What you dont see is that that test has enabled compression for the test and then disabled it again at end of test since host control is allowed and you have asked to tets compression. Hence the 2:1 compression ratio reported during the test.

The messaging could be better during the LTT compression test I guess, but everything is working ok. Host control is working as expected.

Your issue appears to be Linux related and for some reason your system is disabling compression periodically.

Cheers,

Dave Dewar

[Amit Freeman]

LTT still reports a compression ratio of 2.22:1 in that last case? I could have sworn that it reported a 1:1 compression value when I performed it earlier. Hmmm. Well then can anybody give any pointers about what I may be doing wrong? The thing I dont understand is that the backup script hasn't changed. So could it be my HBA? My SCSI cable? should I even look at these?

[Amit Freeman]

Correction. I think I have given some false information here. If I do mt -f /dev/nst0 defcompression -1, LTT used to say that compression was disabled. It no longer does so.