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LTO4 - slow restore from encrypted tape

Tom O'Toole
Respected Contributor

LTO4 - slow restore from encrypted tape


Has anyone experienced anything like this? I'm trying to restore a 380GB volume from an LTO4 tape with encryption enabled (openvms with an MSL4048 library). I have restored smaller things no problem. This restore runs at about 80GB/s (what I'm used to for an LTO4 restore), but after about the 60GB mark, the speed drops dramatically to around 3.5MB/s. I have this restore I started yesterday that's still running.

Yesterday when I checked it, it was running at about 3.5Mb/s and the library front panel said "decrypting". Now this morning it's running at about the same rate, but the front panel is mostly in "idle" with occasional "decrypting".

Haven't had a problem with other smaller restores, or any plaintext restores.
Can you imagine if we used PCs to manage our enterprise systems? ... oops.
8 REPLIES
Tom O'Toole
Respected Contributor

Re: LTO4 - slow restore from encrypted tape


Also, can anyone else comment about what RESTORE speeds they are getting with LTO4 (enc or plain), or for that matter other media?

Everyone always talks about backup speeds, but what about when you really need that tape for a restore?
Can you imagine if we used PCs to manage our enterprise systems? ... oops.
Marino Meloni_1
Honored Contributor

Re: LTO4 - slow restore from encrypted tape

Backup and restore for a Drive hw point of view can reach the same value, inside of the drive the only limit is the rheel motor tape speed
usually what cause througputh limitation is the disk, as source or destination
out of that, also bad tapes, causing lot of rewrite or lot of reread can slow down backup or restore.
In your case, if the drive says it is idle, that mean he is waiting, so waiting to fulfill his buffer during write process or waiting to empty his buffer during restore process.
all seems to point to the disk.

I would suggest, once you finish the restore to collect immediately a support ticket, and attach here, we may be able to see if you suffered read/write issue or if the cause was the data target.
Tom O'Toole
Respected Contributor

Re: LTO4 - slow restore from encrypted tape

Thanks, are you (or anyone) actually seeing 200+MB/s restore figures? I have fully tricked out EVAs with 240 drives and don't ever see more than 80MB/s restores. I do get 200+ on backups.


In this case, I'm pretty sure it's not the disk. The write rate to that volume is averaging 50 operations (64kb size) per second. That's like 3.5 MB/s. According to evaperf there is no queue or unreasonable write latencies on the eva, and the host stats agree, most IOs are getting turned around in <5ms.


I'm used to seeing restore<
Can you imagine if we used PCs to manage our enterprise systems? ... oops.
Dennis Handly
Acclaimed Contributor

Re: LTO4 - slow restore from encrypted tape

>In this case, I'm pretty sure it's not the disk.

One way to prove this is to read from tape and not write to disk. Can you do that?
wobbe
Respected Contributor

Re: LTO4 - slow restore from encrypted tape

I've seen backup and restore times increase dramatically when there are lots of small files stored in a single folder.
Tom O'Toole
Respected Contributor

Re: LTO4 - slow restore from encrypted tape


This restore is about 80 very large files, so file system activity is not the problem. and I will run ltt today probably to get a performance test. But as I said the disk end of it looks fairly idle.

update: I had to run another restore last night of the same stuff from another day, and this one only took 3.5 hours (that's an average of around 30MB/s - not spectacular, but I can live with it).

When the first job finished (in around 24 hours) the backup utility reported 20 SOFTRERRS - recoverable media errors. This seems to be an indication something is up with that particular tape. Occasional recoverable media errors being returned to the backup application (so it could then do a re-read) were normal in 9-track days, but shouldn't be happening with modern tape formats. I'll definitely be following this up with HP.



Can you imagine if we used PCs to manage our enterprise systems? ... oops.
Tom O'Toole
Respected Contributor

Re: LTO4 - slow restore from encrypted tape


It sure would be nice if someone would post what kind of restore numbers they are getting. Does anyone else actually do restores?
Can you imagine if we used PCs to manage our enterprise systems? ... oops.
Tom O'Toole
Respected Contributor

Re: LTO4 - slow restore from encrypted tape


Now I have another tape that's doing the same thing - reading at about 4MB/s. I connected to the web interface and there are no entries in the error log. I killed the backup and restarted, just doing a backup/list to eliminate the disk system. Sure enough, it's the tape.
Can you imagine if we used PCs to manage our enterprise systems? ... oops.