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What do Throughput, Capacity and Compression have in common?

Jaclyn Rothe
Trusted Contributor

What do Throughput, Capacity and Compression have in common?

A: They can all cause back up failures for many different reasons.

Basically a quick summary of this is:
If throughput is below the minimum specs for the drive the drive will stop and start and/or shoe shine. This causes premature wear to drive, but also increases the amount of time for the back up, can cause bad blocks on the media and thusly there will be less space on the tape. Because the compression ratio is calculated on the space available on the tape with the amount of data written the compression ratio will drop.

A production environment can typically expect (for primarily text data) 1.4:1 up to 1.8:1.

Data Set also has everything to do with compression. If the back up has any drawings (CAD files), pictures, PDF docs, video files, these are either already compressed or will not compress. Text compresses extremely well and it is not uncommon to get an all text back up to compress at 2.4:1 ratio.

The best way to prove determine if you have a compression issue with the drive is to run the compression test in LTT and see that the ratio is 2:1 (most of the time will be over 2:1). Then generate a support ticket... and view if there are any errors. Then run a SYSPERF test on a partitioned drive. If you have slow throughput (below drive's minimum spec) this will affect the compression ratio in the back up software.

ALSO, software compression should never be used... It hardly ever gives the best compression ratio.

HP StorageWorks - DLT and Ultrium Hardware vs. Software Compression
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&objectID=lpg50030&jumpid=reg_R1002_USEN

Important things:
Veritas has two places that address compression

On the device properties "Enable Compression" needs to be checked (Ie: selected) or the algorithm in an HP drive will be disabled and the drive will get 1:1 ratio or worse (0.8:1).
From my experience, in the individual job properties (depending on the verison of veritas), anything that has "none" should not be chosen or the hardware compression algorithm in the drive will be disabled.

Some other crucial documents that go hand in hand with poor compression ratios during back ups are as follows:

Tape Capacity Differences Prevent Remote Storage from Making Media Copies
http://support.microsoft.com/?kbid=266011

How File System and Disk Configuration Affect Backup Performance
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&objectID=lpg50032

Performance Assessment Testing:
http://www.hp.com/support/pat

HP Surestore/StorageWorks- Data Compression Information :
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&objectID=lpg50244&prodTypeId=12169&prodSeriesId=42846
8 REPLIES
Jaclyn Rothe
Trusted Contributor

Re: What do Throughput, Capacity and Compression have in common?

*bump*
Steven Clementi
Honored Contributor

Re: What do Throughput, Capacity and Compression have in common?

A. The more you want of each, the more it costs?


;o)
Steven Clementi
HP Master ASE, Storage and Clustering
MCSE (NT 4.0, W2K, W2K3)
VCP (ESX2, Vi3, vSphere4, vSphere5)
RHCE
NPP3 (Nutanix Platform Professional)
Jaclyn Rothe
Trusted Contributor

Re: What do Throughput, Capacity and Compression have in common?

That would also be true :D
Tom O'Toole
Respected Contributor

Re: What do Throughput, Capacity and Compression have in common?


One problem is if you want to encrypt tapes, but don't have drives that support encryption in hardware. Encrypted data compresses extremely poorly. My experience is that slightly better throughput is achieved when drive compression is explicitly turned off.

SDLT320 tape drives can be driven full blast with no hw compression at 18MB/s using OpenVMS backup software AES encryption with alphaserver or integrity systems. The fastest itanium we have here (1.5Ghz/6MB cache) can backup/encrypt at around 31MB/s which is a lot slower than an LTO3 native rate. Possible solutions to stream these drives:

o disk buffer the encryption
o see if the algorithm can/will be
optimized further
o get much faster itaniums
o obtain drives with encryption hardware
o use neoscale or similar appliance

Comments?

Can you imagine if we used PCs to manage our enterprise systems? ... oops.
Jaclyn Rothe
Trusted Contributor

Re: What do Throughput, Capacity and Compression have in common?

LTO 3 native transfer rate is 80mb/s with LTO 3 media, so that would not be true...

Usually you would expect to set a high as possible buffer/block size to ensure you have a steady stream of data.

Utilize a fast RAID array and also HBA SCSI card for maximum throughput and match the tape drive maximum native throughput and burst rate to the hardware you are connecting it with.

IE: If you RAID array is producing 16mb/s you are going to get 16mb/s backups, because your tape drve can NOT possibly out perform the server it is limited to how fast data is transferred to it.

Additionally LTO technology has adaptive tape speed so the tape drive doesn't out perform your system, but instead matches it and attempts to stream data. But is your system performance is low you can forget attaining the maximum performance for the LTO 3 technology... this is also true of any tape drive with high native transfer rate like SDLT, LTO 1 and LTO 2... if your server can't meet hte minimum requirements of the drive, much more is affected then just the time it takes for your backup to run.
Jaclyn Rothe
Trusted Contributor

Re: What do Throughput, Capacity and Compression have in common?

LTO 3 native transfer rate is 80mb/s with LTO 3 media, so that would not be true...

Usually you would expect to set a high as possible buffer/block size to ensure you have a steady stream of data.

Utilize a fast RAID array and also HBA SCSI card for maximum throughput and match the tape drive maximum native throughput and burst rate to the hardware you are connecting it with.

IE: If your RAID array is producing 16mb/s you are going to get 16mb/s backups, because your tape drve can NOT possibly out perform the server it is limited to how fast data is transferred to it.

Additionally LTO technology has adaptive tape speed so the tape drive doesn't out perform your system, but instead matches it and attempts to stream data. But if your system performance is slow you can forget about attaining the maximum performance for the LTO 3 technology... this is also true of any tape drive with high native transfer rate like SDLT, LTO 1 and LTO 2... if your server can't meet the minimum requirements of the drive, much more is affected then just the time it takes for your backup to run.
Jaclyn Rothe
Trusted Contributor

Re: What do Throughput, Capacity and Compression have in common?

I also wanted to add to this thread, that I have seen issues with disabled compression algorithms. This has been across all technologies (DAT, DLT, LTO, autoloaders/libraries) and solely related to Backup Exec.

I have still yet to discover how to prevent this from happening.

Basically, Backup Exec has been designed to favour software compression and as soon as the BUE services start, a command is sent to the HP tape drive to disable the compression algorthim. Becasue of this you can no longer expect 2:1 compression. When this happens I often see 1:1 compression or lower (0.80:1).

We have tried all combinations of drive compression settings and job settings, to no avail. From what I originally thought, if there were numerous jobs scheduled (monday-sunday, monthly, quarterly etc) IF ANY of those had the selection 'use hardware compression if available otherwise none'... or an alternate 'none' setting, the algorithm turned up disabled, so all jobs would need to be changed appropriately. But, I have now also seen this a few times when only one job was configured. Ideally there should be a setting for 'hardware only'... but that does not help the current situation.

If anyone has input how to stop Backup Exec from turning off hardware compression, feel free to post to this thread.
Jaclyn Rothe
Trusted Contributor

Re: What do Throughput, Capacity and Compression have in common?

In addition, in regards to the backup exec issues, I was able to determine that even though an HP tape drive shows in LTT as 'compression disabled' it just means that it is disabled at that moment.

Backup exec still disabled the algorithm and toggles it on an off for backups.... but it is controlled by backup exec and the compresseion raito is still very poor in this circumstance.

Please observe your ratio in the backup job log to deterine what you are achieving with backup exec running the compression.