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15k SAS Speeds

Mornincupofhate

So I've been looking at 15k SAS drives online and I'm looking to see how fast they are MB/s and latency in ms,  but the numbers have been all over the place. 

 

Some sites say internal transfer rate is like 1000MB/s but sustained transfer is 100MB/s now what in the hell does this mean

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4 minutes ago, Electronics Wizardy said:

Don'tbuy them there not that fast, sequentical up to about 200 mb/s and about 5ms of latency in the best case. Really get a ssd, 15k drives are dead.

What if it's for a big data solution?

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2 minutes ago, Mornincupofhate said:

What if it's for a big data solution?

just get a ssd. They were only used in high iops uses before ssds. A single ssd is much better than a large raid 10 of these hdds.

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32 minutes ago, Mornincupofhate said:

So I've been looking at 15k SAS drives online and I'm looking to see how fast they are MB/s and latency in ms,  but the numbers have been all over the place. 

 

Some sites say internal transfer rate is like 1000MB/s but sustained transfer is 100MB/s now what in the hell does this mean

These 15K RPM SAS drives offer speeds in excess of 350 MB/s!

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2 minutes ago, TheCherryKing said:

These 15K RPM SAS drives offer speeds in excess of 350 MB/s!

so an ssd will due 1.5-2 times that and not all reach that. 

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1 minute ago, GDRRiley said:

so an ssd will due 1.5-2 times that and not all reach that. 

Not exactly... It depends which type of SSD you use.

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Just now, TheCherryKing said:

Not exactly... It depends which type of SSD you use.

even a 5 year old ssd will beat it, a pcie or similar will do 5 times read 2-3 time write 

Good luck, Have fun, Build PC, and have a last gen console for use once a year. I should answer most of the time between 9 to 3 PST

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Just now, GDRRiley said:

even a 5 year old ssd will beat it, a pcie or similar will do 5 times read 2-3 time write 

A SATA SSD will be about 1.5 times the speed of the 15K RPM SAS HDD 500 MB/s

A SAS SSD will be about 3.0 times the speed of the 15K RPM SAS HDD 1.0 GB/s

A PCIe 4x SSD will be about 7.5 times the speed of the 15K RPM SAS HDD 2.5 GB/s

A PCIe 8x SSD will be about 15.0 times the speed of the 15K RPM SAS HDD 5.0 GB/s

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15k rpm drives are good for very fast access times to data, small random seek times.

Basically, you have data on some track near the outside of the drive and you have some data on a track near the inside of the drive ... the read/write heads move to the new position to read the track and then the disc has to spin until the data is under the read/write head.

So basically the faster rpm only helps reduce the time required for the whole disc to spin until the data is under the read/write heads.

 

The sustained read or write speed will depend on the data density on the platters (which you'll find in datasheets) and where the data is physically stored on the platters inside the disc. On one edge of the platters you will have the maximum transfer speed - let's say up to 250-350 MB/s - because within one platter rotation a lot of surface and therefore a lot of bits move under the read/write heads. However, as the read/write heads go towards the other edge of the platters, the sustained speed will gradually go down, and it wouldn't be uncommon to have speeds of 100 MB/s or even less. 

This is one of the reason sometimes people used to buy big capacity drives (like 75 GB 15k rpm drives) and create a single 15-25 GB partition on it, because this way, the read/write heads would be forced to stay only within one area of the platters where the transfer speed is consistently high.  This technique was/is called "short stroking".

See http://www.tomshardware.com/reviews/short-stroking-hdd,2157.html or the video at the bottom of this post

 

The datasheets for various drives will also list that higher speed like 1000 MB/s ... what does that mean.. well think of it like this ... the hard drive has some cache memory on it, let's say 16 MB to 128 MB.  If some application requests 100 KB from the hard drive (let's say a jpg picture), the hard drive will move its read/write heads to position, wait until the platters spin around and data goes under the read/write heads and the hard drive reads the 100 KB of data... but since it's already there, it will continue to read the data track and put the information in the hard drive cache. This way, if by some accident the application will then request another amount of data from the hard drive and that data just so happens to have been stored after the original data, the hard drive already has that in the cache and can serve it without moving read/write heads, waiting for platters to complete a rotation and so on.  So basically the high speed just tells how fast can the controller read the data from internal caches and send it through the cables to the computer.

In real world. it's not really that relevant.

 

So yeah, basically the 15k rpm drives were trying to solve an issue.. when databases had to query lots of small chunks of data spread on various parts of the hard drive and there was no enough ram memory to cache the tables in memory (or the data was updated too often to be possible to cache in memory).

In such cases, the hard drives would have to constantly move their read/write heads to access the data or overwrite existing data. These drives were almost never used for their capacity but rather for short access times and high I/O.

 

SSDs solved these issues... any flash memory chip can be accessed just as fast as the others, and usually extremely fast (let's say within 0.1 ms compared to around 3-5ms average seek times on 15k rpm drives). Also, modern SSDs can generally saturate the interface they're connected to, at least when it comes to reading data from them. For example, most SSDs can easily reach around 550 MB/s read speeds, which is about as fast as the SATA 3 (6gbps) can do.

There's also m.2 SSDs which can connect using 1, 2 or 4 pci-e lanes, where each pci-e lane can transfer  500 MB/s or 980 MB/s (pci-e v2.0 versus pci-e v3.0) .. so in theory a single SSD has the potential to read or write at up to 4 GB/s

 

There's just a tiny "downside" if you could call it that.  Mechanical hard drives have the tendency to not fail instantly (turn into bricks), usually you have some advance warnings in the SMART data when a hard drive may fail. Also often you can tell by the degraded performance or by noise. So, if you're careful you can usually spot a mechanical drive before it fails completely.

With SSDs however, a large majority of failures are more or less instantaneous and data is harder to recover from them.

 

BUT, if you have two SSDs in RAID 1 (one ssd is mirror of the other), or maybe at least 4 SSDs in a RAID 6 (data can be recovered even if 2 ssds fail completely), you would kill those 15k rpm drives, even if you'd have a RAID 10 or some other expensive setup.

 

Hope it helps

 

// the video i mentioned , it's from 2014 so you can sort of figure how old these techniques are

 

 

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