I was wondering why do servers still come with SAS disks instead of SSD disks? I know that SAS are faster than normal hard drives but they are still much slower than SSDs. I think they are more expensive too :s
so what's the deal here?
Cost, capacity and reliability are factors for why SSD adoption hasn't occurred at all levels. SSDs cost more than SAS disks for a given capacity.
But in general, servers don't actually come with a particular type of disk. Storage is something that is configured afterwards.
Some background information: Are SSD drives as reliable as mechanical drives (2013)?
Also note that not every workload benefits from SSD. Provisioning 20TB of bulk storage where only a small subset needs to be active at a given time is best for standard disks (plus tiering/caching). Purely sequential workloads are better served by spinning disks. And more than that, SSDs tend to do best outside of the disk form-factor. For the past year, I've been using PCIe-based SSDs to get around cost, performance and compatibility issues with traditional disk-based SSDs.
In many cases, enterprise storage frames actually employ three different kinds of disks- SSD, SAS and SATA (or LSAS drives which are SATA with a SAS controller), with the goal being to optimize the data being stored with the I/O it actually needs. In other words, data accessed frequently ends up living in the highest speed areas (flash) and data accessed very infrequently lives on lower speed drives (spinning media), all connected to the same array. Should a file suddenly become highly in demand, the storage frame moves it to a higher speed section of the frame to assist in user access. When demand dies down, it slips back to the middle tier or the archival tier.
Part of the reason to do this is cost savings on expensive fast drives, but also it simplifies storage management and helps to provide better service to your end users.
Many servers still use spinning disks with a SAS interface, but most (all?) servers you buy these days should have options for SSDs as well.
The price comparison you're making is a little tricky - an enterprise SAS disk is definitely more expensive than most consumer SSDs, but an enterprise SSD is a whole lot more expensive than a consumer one - they're still pretty costly to equip a server with, and some workloads don't need the extra performance.
So, for the larger question of "why haven't SSDs taken over the data center?", the answer is that they're getting there but the cost is still too high to replace spinning disks for all workloads.