Scsi

[forumdude123]

When i was looking for a scsi card online, it said that one scsi port can support up to 15 devices!! Can someone please explain this to me, because i don't get this at all. Thank you.

[bobbyraw]

First of all scsi was designed for MAC, when they reffer to to supporting up to 16 devices they are referring to ZIp drive, Cdrom, Scanner,hard drive and so on, all chained together. internally or external. for example you have a scsi port with a scanner connected you would connect the cdrom to the scanner and the zip drive to the cd rom. and at the same time a ribbon from the card to you internal components ( hard drive). so basically one device is physically connected to the scsi port. and the rest are chained through each other. and each device MUST be given an ID number. This can only be done with wide scsi for anything over 8 devices.

[forumdude123]

First of all scsi was designed for MAC, when they reffer to to supporting up to 16 devices they are referring to ZIp drive, Cdrom, Scanner,hard drive and so on, all chained together. internally or external. for example you have a scsi port with a scanner connected you would connect the cdrom to the scanner and the zip drive to the cd rom. and at the same time a ribbon from the card to you internal components ( hard drive). so basically one device is physically connected to the scsi port. and the rest are chained through each other. and each device MUST be given an ID number. This can only be done with wide scsi for anything over 8 devices.

does this mean i can run 12 hard drives, and 3 optical drives from that one port? (I'm just interested, i'm probably never going to use more than 3 hard drives!!!)

[bobbyraw]

yes you can, but anything over 8 would have to be on a WIDE scsi. and these cards cost a bit too

[jbennet]

most servers have SCSI but SATA is becoming more popular for smaller servers

[forumdude123]

which is faster?

[jbennet]

nowadays the fastest SCSI is only slightly faster than SATA 2 but its a lot more expensive. Its double the speed of SATA 1 though, and many many many times the speed of IDE, making it still a very good choice for servers, but not as good as it once was.

SCSI supports much more devices on one cable though. SATA needs individual cables which means that in serversd which can have ~16 drives , SCSI is a better choice. Also, many existing devices e.g expensive tape drives are SCSI.

[DimaYasny]

just read the wiki articles, everything is explained.

the more devices you use on a SCSI the slower they are, because the bus is shared.
for better performance use SAS. It is partially compatible with SATA, and you can connect about 128000 devices to it. I have tried 60 HDDs - works faster than WIDE SCSI with 14 drives on it.

btw the latest SCSI contollers, before the transition to SAS were able to do 640Mbps and supported 32 devices (the card and backplane included)

[jbennet]

my compaq proliant server has 8 scsi disks on it and is running fine (has been up since oct 2001, runs redhat 7)

[DimaYasny]

I've recently set up an array of 26 SAS drives for a bank in the UK - the access times to the SAS array is faster then the access time to their server internal SCSI drives.

that doesn't mean the SCSI drives are slow, that means SAS is faster

[jbennet]

yeah i know

my server has a fibre channel card but ive never used it - i think its for stuff like that

[DimaYasny]

no, the HBA is there for external connections to a SAN. And you generally should have two for channel redundancy

[jbennet]

oh i see (i have one?)

never used it so meh

[DimaYasny]

LOL
if I had a proper server for my own use, I'd set it up with ESX or XEN and run a whole network in it :)

[jbennet]

i got it free from somewhere that was binning it (quad pentium 2 233mhz and a gig of ram) - ran NT4/Backoffice but then i put redhat on it and it does SAMBA now - stores all my music in a mirrored raid array

[DimaYasny]

mirror on 8 disks?

[forumdude123]

i guess sataII is the way to go!!

[jbennet]

SATA is a better choice for desktops due to it being cheaper
SCSI is better for servers due to expadability

[DimaYasny]

SATA is just as expandable as SAS, and SAS is much more expandable than SCSI

[jbennet]

2 disks are O/S - 1 = swap, 1 = system

the system is mirrored onto disk 3
no point mirroring swap

leaving disks 4-8 as data drives - 2 are mirrored onto the other 2

[DimaYasny]

the normal server configuration - Raid1 for OS, Raid5 for Data and 1 global hotspare

[jbennet]

all my drives are hotswap (they slot in the front on quick release things) so i can just pop out failed ones and out new ones in if i need too - hence the uptime of over 7 years

[DimaYasny]

you are confusing hotswap with hotspare. a hotspare is a drive that is idle, and if a disk fails in a raid array, this drive jumps in to take it's place.

[forumdude123]

you are confusing hotswap with hotspare. a hotspare is a drive that is idle, and if a disk fails in a raid array, this drive jumps in to take it's place.

but wouldn't the hotspare fail almost as quickly as the other disks since it's exposed to the same temperature conditions as the other disks!!

[DimaYasny]

who said disks fail only because of temperature conditions? that's only one reason, and definitely not the main one

[jbennet]

mine generally fail due to physically wearning out - i have dismantled two and they were covered in dust and the glass platters had areas which had no magnetic coating left , just polished glass.