topic Why does VA 7xxx use more space when formatting disks in Disk Enclosures

Why does VA 7xxx use more space when formatting disks

Tim D Fulford — Wed, 19 Jun 2002 07:14:49 GMT

Hi

The above question needs a bit of an explaination. We will be soon moving over to VA7400 as a replacement to a 30 disk fc60 array. The basic unformatted disks seem to be similar in size 18, 36, 73 (see http://www.hp.com/products1/storage/products/disk_arrays/midrange/va7400/infolibrary/product_brief.pdf). I've been told that the VA will format in RAID 1 two 18 GB (unformatted disks) to 16GB for LVM, where as the fc60 (and JBOD, internal disks) go to 17GB. Why is this the case? Or have I been misinformed?...

I've been scouring the web for the armcfg, armdsp, armmgr... commands in the hope they may shed some light on the subject but I can't find any trace of these either!!!

Regards

Tim

Re: Why does VA 7xxx use more space when formatting disks

harry d brown jr — Wed, 19 Jun 2002 10:33:11 GMT

Tim,

Here is a white paper on the math behind the scenes:

http://www.hp.com/products1/storage/products/disk_arrays/infolibrary/mathematics_behind_RAID_5DP.pdf

live free or die
harry

Re: Why does VA 7xxx use more space when formatting disks

Tim D Fulford — Wed, 19 Jun 2002 10:38:23 GMT

Harry many thanks. I've read the white paper but RAID5DP is not RAID1... I'll re-read it anyway 7pts for diligence...

Tim

Re: Why does VA 7xxx use more space when formatting disks

Vincent Fleming — Wed, 19 Jun 2002 10:45:48 GMT

There are several reasons for the usable capacity to vary a little from the marketing blurb...

First, the drive sizes are approximate, and based on the manufacturer's labeling. FOr example, the 73GB drive might actually be 72.5GB. Sometimes they're a little smaller, sometimes a little larger.

Second, the capacity listed is based on a 512-byte block size when formatted. The VA formats them at 520-byte blocks so that it can store additional parity information in every block (it adds an additional CRC to each and every block to ensure data integrity). This also lowers the useable capacity.

The FC60 used different drives, and a different block size, which would account for the slight difference in usable capacity.

To find docs on the arm commands, pull down the .PDF of the Command View SDM users' Guide from www.hp.com - they're in the back.

Good luck!

Re: Why does VA 7xxx use more space when formatting disks

Tim D Fulford — Wed, 19 Jun 2002 11:05:59 GMT

Vincent. I also took a "quick" look at this guide too yesterday. I re-read it also. Many thanks for the info about the block & extra CRC stuff, this probably accounts for the difference (I'm pritty sure [but have no proof] the disks are the SEAGATE ones).

Harry & Vincent, as I was re-reading the Maths...RAID5DP paper I ended up going to the white paper on VA one section caught my eye which hints at what Vincent was saying "Data Integrity & High Availablity Fearures".... I quite like the white papers but wish I could use an edit switch to get rid of marketing guff & turn on explain more mode ...

Many thanks 10pts to Harry for the explaiation about block size & extra CRC. quick calculation 16.5 == fc60 assuming 512b is data in 520b block (data+CRC) ... 16.5*512/520 = 16.24, so the maths seems to work out

If any one has any more comments about VA7400 feel free to have your say...

Tim

Re: Why does VA 7xxx use more space when formatting disks

Carlos Fernandez Riera — Wed, 19 Jun 2002 11:17:36 GMT

See : http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x1aa20cb17a32d5118fef0090279cd0f9,00.html

You must add (substract) also LVM headers, that are larger as larger are disks.

Re: Why does VA 7xxx use more space when formatting disks

Tim D Fulford — Wed, 19 Jun 2002 11:41:19 GMT

Carlos, Thanks, LVM headders were always there in fc60 & internal disks, besides they are only 4MB (if PE==4MB) which is some way off 400MB or so. 7pts for participationg

Vince, I've looked for the HP Command... & can't find it. It turns out yesterday I was actualy looking at Installation guide for VA7xxx (or words to that effect), it just mentions SDM... If I do a search in HP for "HP Command view SDM Installation and user guide" (as suggested in the Installation guide for VA7xxx....) I gets lots of docs but none are the .pdf I want.

I'll keep digging, unless someone can give me the URL (10 whole points at stake here!!!)

Tim

Re: Why does VA 7xxx use more space when formatting disks

Vincent Fleming — Wed, 19 Jun 2002 11:49:54 GMT

To find it, go to www.hp.com, then Products and Services, Storage, Disks and Disk arrays, Modular Storage Spftware, HP Command View SDM, Support for your product, Command View again, and then manuals.

You can find manuals under "Support for your product" for all products. (at least most!)

Here's the URL:
http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&locale=en_US&prodTypeId=12169&prodSeriesId=64112&prodSeriesName=hp+surestore+command+view+sdm&taskName=troubleshoot+a+problem&taskId=8413&prodSeriesId=64112&docIndexName=manuals

There are 2 manuals there. The command 'man' pages start on page 84 of the Installation and User Guide.

Good luck!

Re: Why does VA 7xxx use more space when formatting disks

Tim D Fulford — Wed, 19 Jun 2002 13:35:27 GMT

Vincent,

Congratulation one becomming a Wizzard.... I think I just tipped you over the 1,000 pt mark!

Tim

Re: Why does VA 7xxx use more space when formatting disks

Vincent Fleming — Wed, 19 Jun 2002 14:11:53 GMT

Thanks Tim... but I'm not there yet. Close, though. Please no points on this one - I'm happy to earn the points. Good luck with the VA!