Some backup questions
Posted: Mon Oct 23, 2023 1:42 pm
I recently finished archiving roughly 24TB of data to tape (fodder for another discussion). My tape drive (and disks) are connected via fibre channel. The tape model is LTO7 which has a native transfer speed of up to 300 MB/sec and a native capacity of 6TB.
My Itanium system can write to tape a little faster than the Alpha but my initial attempt couldn't write much faster than about 80 MB/sec and consumed most of the CPU. Turning off CRC more than doubled the write speed to around 184 MB/sec while consuming a fraction of the CPU.
Question 1: does the CRC routine used by backup (I am assuming LIB$CRC) use hardware CRC instructions? If not, can it be changed to do so?
I was using a block size of 65024. Writing that block size at 184 MB/sec means writing just under 3000 blocks a second (assuming my math is correct). Running a loop of LIB$CRC that many times in a test program gave the following completion times:
DS15 @ 1GHz: 1.29 secs
RX2620 @ 1.4GHz: 1.42 secs
x86 @ 3.2GHz: 1.11 secs
So, if it takes longer to compute CRC than it takes to write the block out, it is not surprising that a backup with CRC outputs at less than 1/2 the speed of a backup without it. Hopefully, something can be done about this.
Question 2:
As mentioned, I was rolling out a 24TB volume set. When checking progress with control-T I noticed that the amount saved would wrap at 2TB would indicate that this variable is being truncated or masked to 32 bits on output. Any chance of changing this to 64 bits?
Question 3:
Backups of this size currently take multiple days. My x86 system is faster than my other systems in every way and I am hoping that a backup using it will come closer to the 300 MB/sec speed the tape drive is capable of than the systems that currently have SAN tape support. How soon can we expect to see SAN tape support on x86?
Thank you.
My Itanium system can write to tape a little faster than the Alpha but my initial attempt couldn't write much faster than about 80 MB/sec and consumed most of the CPU. Turning off CRC more than doubled the write speed to around 184 MB/sec while consuming a fraction of the CPU.
Question 1: does the CRC routine used by backup (I am assuming LIB$CRC) use hardware CRC instructions? If not, can it be changed to do so?
I was using a block size of 65024. Writing that block size at 184 MB/sec means writing just under 3000 blocks a second (assuming my math is correct). Running a loop of LIB$CRC that many times in a test program gave the following completion times:
DS15 @ 1GHz: 1.29 secs
RX2620 @ 1.4GHz: 1.42 secs
x86 @ 3.2GHz: 1.11 secs
So, if it takes longer to compute CRC than it takes to write the block out, it is not surprising that a backup with CRC outputs at less than 1/2 the speed of a backup without it. Hopefully, something can be done about this.
Question 2:
As mentioned, I was rolling out a 24TB volume set. When checking progress with control-T I noticed that the amount saved would wrap at 2TB would indicate that this variable is being truncated or masked to 32 bits on output. Any chance of changing this to 64 bits?
Question 3:
Backups of this size currently take multiple days. My x86 system is faster than my other systems in every way and I am hoping that a backup using it will come closer to the 300 MB/sec speed the tape drive is capable of than the systems that currently have SAN tape support. How soon can we expect to see SAN tape support on x86?
Thank you.