Thanks for your reply. do you mean that I will see the behaviour you're describing only if I'm reusing tapes with hardware-compressed data? I only have 3 tapes so far, so don't mind erasing them.
I find this slightly confusing. My understanding at this stage is that as long as I start with "fresh" tapes, and then use mt -f /dev/nst0 defcompression 0 to switch off hardware compression, I should be OK from then onwards? Surely the argument about the tape drive switching into compression mode (when fed a hardware-compressed tape) then also works the other way round, i.e. when a tape drive has hardware compression enabled, and is fed a non-hardware compressed tape, then it won't enable hardware compression? I see that you're saying use stinit, but I've had a look at that and at this stage it will only add more complexity to (for me) a rather complex situation. If I could get away with erasing 3 tapes, using mt -f /dev/nst0 defcompression 0 (maybe add that into crontab for good measure), and it all works reasonably well, then I'll be happy for now. Will appreciate your thoughts and thanks a lot for being so helpful. Joe Jon LaBadie wrote: > > On Wed, Jul 05, 2006 at 02:42:14AM -0700, Joe Donner (sent by Nabble.com) > wrote: >> >> Thanks to everyone for your replies. >> >> Ok, so what I really wanted to do was to use software compression, and >> not >> hardware compression. >> >> Cyrille's suggestion sounds like what I need, but again I'm not sure >> whether >> using this command will mean that compression is turned off >> "permanently". >> I'll look into the stinit command in the meantime... >> >> Now I'm wondering about the tapes I've already used while hardware >> compression was still on. Obviously the tape drive will need to feed >> those >> tapes through its decompression mechanism to read them again. If I use >> the >> command as suggested by Cyrille, will that mean that the used tapes >> become >> unreadable, or that you have to manually turn compression on and off >> (I've >> read the the compression command overrides the defcompression one for the >> currently loaded tape)? >> >> Thanks. >> >> Joe >> >> >> Cyrille Bollu wrote: >> > >> > [EMAIL PROTECTED] a ?crit sur 04/07/2006 13:17:46 : >> > >> >> >> >> Dear all, >> >> >> >> I just want to clarify something about compression: >> >> >> >> My understanding is that you can use either software or hardware >> >> compression, or can switch between the two if needed. >> >> >> >> What is generally, in your experience, the best of the two to use? >> >> >> >> To switch off hardware compression, I believe I should use: >> >> mt -f /dev/nst0 compression 0 >> >> >> >> Does this command switch it off permanently until you use the >> > following?: >> >> mt -f /dev/nst0 compression 1 >> >> >> >> or will hardware compression switch back on, say, after a reboot? >> > >> > AFAIK, there's another option. >> > >> > from "mt" man pages: >> > >> > defcompression >> > (SCSI tapes) Set the default compression state. The >> value >> > -1 >> > disables the default compression. The compression state >> > set by >> > compression overrides the default until a new tape is >> > inserted. >> > Allowed only for the superuser. >> > >> > so one should use: >> > >> > mt -f /dev/nst0 defcompression 0 >> > > > > Some posters have noted that in their environments when the tape drive > reads data that was HW compressed, it resets itself to HW compression. > This greatly impacts your situation if your drive does the same thing. > > The first thing most (all?) amanda commands in preparation for writing > is open the tape, read the tape label to confirm it is an amanda tape > (and the expected one), then close the tape before reopening it again > for writing. Thus, unless you can interpose a "turn off HW compression" > between the close and reopen, the drive will be in HW compression mode > again. The only way(s) to force the HW compression off at each opening > is with stinit or perhaps Cyrill's defcompression. > > Gene Heskett has posted a number of times the steps he uses to ensure > tapes previously used with HW compression won't trick the drive. It > basically is rewind, close the drive, force HW compression off, use > dd to write a lot of data to the beginning of the tape (20-100MB IIRC). > > If needed you can check the list archives at yahoo or marc for his posts. > > -- > Jon H. LaBadie [EMAIL PROTECTED] > JG Computing > 4455 Province Line Road (609) 252-0159 > Princeton, NJ 08540-4322 (609) 683-7220 (fax) > > -- View this message in context: http://www.nabble.com/Compression-usage-tf1889370.html#a5181674 Sent from the Amanda - Users forum at Nabble.com.
