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Disks and Things

2. Disks and Things. Introduction. This is a mix and match section It contains all the information that does not have a home elsewhere in this presentation It covers Disk layouts Disk Farms Striping on Striping. Disk Layouts.

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Disks and Things

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  1. 2 Disks and Things

  2. Introduction • This is a mix and match section • It contains all the information that does not have a home elsewhere in this presentation • It covers • Disk layouts • Disk Farms • Striping on Striping

  3. Disk Layouts • In IQ (Simplex or Multiplex) the disk performance is critical to the overall speed of the system • In an IQ-M system you can have a number of different disk configurations • We consider the following • Simple SCSI (or UDMA-[1]66) Disks • Basic Disk “Farms” (Collections of raw disks) • RAID Systems (NT RAID, Clariion etc.) • Disk Subsystems (EMC, HDS[SunStor].)

  4. Basic Disk Farms • Here we are considering a number of “dumb” disks • There is no intelligence in the disks – or the controllers • Here is where we use IQ-M striping • Here we have a number of disk drives (on various controllers) • We stripe across a series of disks

  5. Disk Striping Basic Disks 1-dimensional IQ Striping In the 1-dimensional striping we are spreading the data writes (and reads) across a series of disk drives – limiting the overall disk head movement.

  6. Problems with Disk Striping • IQ-M has to make location decisions as to where to write and read the data • It is not as simple as writing to “bit bucket” or a serial list of disk blocks • It is faster than simple 1 disk = 1 device • The performance hit is measured in extra micro seconds per read/write – but with a 10 TBytes database this could be important

  7. In RAID 1 – Mirror, we can consider that Disks A and B are mirrored and Disks C and D This improves read and write performance, as the controller will pick which disk heads are closer to the data required. Also there is an improvement in data security – 2 copies A D A D C B C B RAID – 0,1 Redundant Array of Inexpensive Disks Raid 1 - Mirror In RAID 0 – Stripe, we can spread the reads and writes as per the last slide Raid 0 - Stripe This dramatically improves the read and write performance, but does nothing for the data security

  8. In RAID 0/1 – Stripe and Mirror, we can see that the stripe sets on A and B mirror to C and D This gives security – in the mirroring (copies) of the data, and improved performance due both to the two copies and the stripe sets A D A D C B C B In RAID 5 – Csum/MR, The data is in “stripes” across the disks then a checksum is written to the final disk This gives a small performance improvement for reads, no improvement for writes (in fact it will slow writes down!). The only bonus is security. The system will allow a disk failure in the pack without requiring the 2 to 1 overhead of mirroring systems RAID – 0/1 and 5 Redundant Array of Inexpensive Disks Raid 0/1 – Stripe and Mirror Raid 5 – Checksum Multi-read

  9. Other RAID • There is RAID 2, 3 and 4 • These are variations on the mirroring/striping and checksum theme • There are also proprietary RAID schemes • RAID S The is EMC’s RAID system • RAID M This is the MTI RAID Scheme • We will come on to talk about proprietary schemes in a few slides time

  10. Well is RAID useful ? • Yes it is – mainly ! • If the RAID system handles the data distribution at the hardware level (with RAID controllers) then RAID is both fast and safe • Windows NT (and 2000) can drive RAID through the operating system • This can be slower than IQ-M performing the striping • Watch out for large numbers of reads – and individual read performance slowing down • RAID 5 on Disk Subsystems is a good compromise between speed and reliability

  11. Disk Subsystems (emc, HDS) • These are disk arrays where we have no idea what the internal organization of the disks is (and nor do we care) • Generally we can consider that we are writing to memory (and usually we are!) • The tuning of the disks is usually best left up to the Hardware Support guys

  12. Striping on Striping • There is one very contentious area in disk organisation • This is 2 dimensional disk striping or striping on stripes • This is driven by using an operating system or RAID unit to provide 1 dimensional striping on a series of disks • Then we apply IQ striping onto the existing striping • 2 Dimensional stripes • Do it right – it’s very fast, do it wrong and it isn’t!

  13. A diagram Operating System Striping AS IQ-M Striping The reason this works is now the individual disks only have a very small amount of data to read and write so we rely on very fast disk -> processor communications If this is done properly it is the fastest disk access for ASIQ-M

  14. Disk Striping • By default Disk_Striping is set ON for RAW devices and OFF for file system devices • We should be using RAW devices – they are (generally) a lot faster and potentially safer than file system devices • You may want to play with this parameter if you are running devices on disk farms or RAID array systems

  15. Disk Striping Packed • This was put in to the 12.4.3 release – but is little used. This option changes the number of Allocation Maps from 1 to 16 - allowing each allocation map to present space in specific unit values, such as 1 Block, 2 blocks etc.. • This allows for very slightly slower allocation of pages (minimal incremental cost), but better space utilization within the database – and in some case potentially faster allocation of space if there is a very fragmented store. • It is recommended that this is turned on at all times. • We will talk more about this during CREATE DATABASE

  16. IQ Store Dbspace Management • New functionality permits removing a dbspace from the IQ Store • without interrupting database activity • You can add a new dbspace and move data from an 'old' disk to a new device • Drop a dbspace after all objects have been relocated • New Commands and Functions: • Determine database objects on a dbspace • Set a dbspace in 'read_only' status to prevent new writes • Move objects from one dbspace to another

  17. IQ Store Dbspace Management • Dbspaces may be enlarged • For dbspaces created using 12.6, you may be able to enlarge a dbspace • In order to enlarge a dbspace you must 'reserve' space for the freelist when you create a new dbspace • Explanation follows

  18. Freelist Pages Sybase IQ Dbspace Basics • When you create a dbspace some pages are reserved for the 'freelist' • The freelist tracks pages that are available to hold data in that particular dbspace • The amount of space reserved for the freelist depends on the size of the dbspace

  19. Freelist Pages Reserved Pages Reserving Space for the Freelist • New parameter in the Create Database andCreate Dbspace commands allow for reserving space in the freelist • Command below creates a new dbspace reserving 5 MB for the freelist • Create dbspace newdbs as '/data/iqstore2' IQ STORESize 500 Reserve 5

  20. Alter Dbspace Command • With space for the freelist reserved you could increase the size of the dbspace using the Alter Dbspace command - • Alter dbspace newdbs • Size 750 MB • New Options for dbspace • READWRITE • READONLY • RELOCATE

  21. Dbspace “Commands” • sp_iqdbspaceinfo • What is located within the dbspaces • sp_dbspace • Usages and properties of the dbspace • sp_iqrelocate • Moves data between dbspaces

  22. Disk etc. - End

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