Erwin Craps - IT Helps
Erwin.Craps at ithelps.be
Thu Feb 12 01:52:02 CST 2004
RAID 5 is slower writing than a plain disk. It is advised not to use RAID 5 for databases files (SQL). The reasons for this is that RAID 5 does not only have to write but also calculate a parity bit each time you want to write. It's this calculation that slows down the writing to the disk. RAID 5 is also much slower when a disk of your set has crashed. For the same reason. When one of your disks has crashed it neads to calculate missing bit based on the parity bit each time you read. This is the general rule. There are however WRITE-CACHE available for hardware RAID controllers (as I have) but due to the fact this uses a buffer (128MB READ, 128MB write in my case) I'm supose, but not sure, some of that calculation delay if neutralized by the buffer. But again also in this case RAID 1 would be far superior in writing speed. So it is far better to put databases on to a mirrored disk set (RAID 1) if you want redundancy. The reason why SCSI is still far superior than ATA or SATA drive are mainly these. -No limit in the number of disks (as 4 in ATA or SATA) -Simultaneous access of ALL disks (2 for ATA, don't really know for SATA but I supose also 2) So if you build a mirror with ATA you must pay attention the two disk are on seperate controlers and are not in a master slave relation. Master slave means that the OS can only access one disk at the time. So if you create a mirror on two disks that are in master/slave your mirrored writes will be done sequential and not parralel.. This will dramaticaly slown down your writings. Same thing creating a RAID 5 of 3 or 4 ATA disk. The OS only access 2 disks simulteneous, so you will get a drop in reading/writing. HOWEVER. There are companies that have RAID5 ATA/SATA controllers like adaptec. I supose that hardware RAID controllers using ATA or SATA have solved this issue. Note that a hardware RAID solution is always far superior than a software solution (altough I know 1 good advantage about software mirror). A hardware RAID solution spoofs the Os on a hardware level telling there is only one disk (example) and it should be considered that way. Meaning that the hardware controller probably solved the limitation in some way. But that I don't know. I'm one of those guys that will probably never use ATA or SATA for a server (unless a webserver with plenty memory). On the other hand, I'm buying piece by piece for my home music computer to have a RAID 5 with four 250GB SATA disk. I now have around 300GB of music and video files on my disks and I'm getting pretty scared because there no way I can backup this in a cheap way. AndI'm only half way putting my music collection on disk!!! So four 250GB SATA drives in a RAID 5 leaving me 750 GB would be nice.... Erwin -----Original Message----- From: dba-tech-bounces at databaseadvisors.com [mailto:dba-tech-bounces at databaseadvisors.com] On Behalf Of Drew Wutka Sent: Thursday, February 12, 2004 3:07 AM To: Discussion of Hardware and Software issues Subject: RE: [dba-Tech] Win2k Server and RAID Not correct. You can Mirror the OS 'volume' in Windows 2000 server, using the software RAID. It just can't be part of a RAID 5 volume. Go into disk management, make the main drive, and your 'second' drive 'Dynamic Disks'. It will require a reboot. Once that is done, simply right click on your root partition, and tell it to Add Mirror. Piece of cake. As for your earlier question for performance issues, you won't really see any performance drop, but you will get performance gain on drive access. Mirrored will read twice as fast, striped will write twice as fast, and RAID 5 will read and write faster. Drew -----Original Message----- From: dba-tech-bounces at databaseadvisors.com [mailto:dba-tech-bounces at databaseadvisors.com]On Behalf Of John Bartow Sent: Wednesday, February 11, 2004 2:03 PM To: Discussion of Hardware and Software issues Subject: RE: [dba-Tech] Win2k Server and RAID William, My understanding is that when using software RAID 1 Windows OS can not be on a mirrored drive. When using hardware RAID 1 it can. is this correct? John > -----Original Message----- > From: dba-tech-bounces at databaseadvisors.com > [mailto:dba-tech-bounces at databaseadvisors.com]On Behalf Of William > Hindman > Sent: Wednesday, February 11, 2004 1:24 PM > To: Discussion of Hardware and Software issues > Subject: Re: [dba-Tech] Win2k Server and RAID > > > > ...it takes up cpu cycles ...but if your server is primarily a file > server rather than applications oriented, the added cycles won't be noticed. > > William Hindman > Government is not reason, government is not persuasion, government is > force. It is a dangerous servant." G. Washington > > ----- Original Message ----- > From: "John Bartow" <john at winhaven.net> > To: "_DBA-Tech" <dba-tech at databaseadvisors.com> > Sent: Wednesday, February 11, 2004 12:25 PM > Subject: [dba-Tech] Win2k Server and RAID > > > > Does configuring RAID 1 with Win2k Server without a dedicated RAID > > card > have > > any major drawbacks? > > > > TIA > > John > > > > > > _______________________________________________ > > dba-Tech mailing list > > dba-Tech at databaseadvisors.com > > http://databaseadvisors.com/mailman/listinfo/dba-tech > > Website: http://www.databaseadvisors.com > > > > > _______________________________________________ > dba-Tech mailing list > dba-Tech at databaseadvisors.com > http://databaseadvisors.com/mailman/listinfo/dba-tech > Website: http://www.databaseadvisors.com > > _______________________________________________ dba-Tech mailing list dba-Tech at databaseadvisors.com http://databaseadvisors.com/mailman/listinfo/dba-tech Website: http://www.databaseadvisors.com _______________________________________________ dba-Tech mailing list dba-Tech at databaseadvisors.com http://databaseadvisors.com/mailman/listinfo/dba-tech Website: http://www.databaseadvisors.com