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How fast does a SSD need to be?

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Old 10-09-10, 01:49 AM Thread Starter   #1
Ba!nesy
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How fast does a SSD need to be?


I know its normally slower to write to a SSD than a HDD, but at what read speeds does it need to be before it becomes faster than a Sata HDD @ 3gb's?
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Old 10-09-10, 03:16 AM   #2
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An SSD needs a read speed of ~150MB/s to be as fast as the best 3.5" hdd. Where ssd's really shine is in access time and IO/s. Almost all SSD's will be faster than a 3.5" SATA hdd in every way except perhaps write speeds.

The interface speed (you referred to 3.0Gb/s) is the bus speed (SATA 2), which can be misunderstood. The reason is it refers to gigabits per second and generally read and write speeds for storage devices refer to megabytes per second. 1 gigabit = ~120 megabytes, so 3.0 Gb/s = ~360MB/s. Which means for a storage device to saturate the bus would require around 360MB/s of throughput. The new SATA 3 (6.0Gb/s) standard has a maximum bus throughput of around 600MB/s.

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Old 10-09-10, 04:50 AM   #3
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The thing is that the write speed of hard drives depends on which part of the drive your writing to. While the fastest part of the fastest desktop hard drive may be over 150MB/s, the average will be lower, somewhere around 125MB/s. For a normal drive the average write performance is around 100MB/s. These numbers are all 3.5" drives. If you want to look at 2.5" laptop drives the write speed are substantially lower, with averages around 70MB/s.
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Old 10-11-10, 03:44 AM Thread Starter   #4
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Talking about access time and botting up. is the access time about 2 ms per file for an SSD and about 8ms for a HDD? meaning with hundreds possibly thousands of files (i'm guessing here, not actually sure how many files are loaded at bootup) a SSD would be roughly 6 ms faster per file?
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Old 10-11-10, 04:42 AM   #5
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Quote:
Originally Posted by Ba!nesy View Post
Talking about access time and botting up. is the access time about 2 ms per file for an SSD and about 8ms for a HDD? meaning with hundreds possibly thousands of files (i'm guessing here, not actually sure how many files are loaded at bootup) a SSD would be roughly 6 ms faster per file?
Not really, no. The random access time of a normal hard drive is usually something like 10-12 ms. But this depends on where the next file to access is on the hard drive. If it is physically next to the current one the access time is like 0.2 ms but if it is on the other side of the drive the access time can be up to 50-100 ms. The average ends up being around 10 ms. Compared to that a SSD has a access time or about 0.1 ms (two orders of magnitude faster) for pretty much any file.

Access time only measures how long it takes till you start getting data. You then have to add in how long it takes to read all the data you need from the file, which is where the read speeds come in.

Then there is the fact that windows doesn't actually load files. Windows is 10-15 GB and a loaded windows installation takes less than 500 MB of memory. The way windows does this is that rather than loading full files it only load the bits and pieces of a file it need.
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Old 10-11-10, 06:12 AM   #6
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Newer SSD's such as the Vertex 2 are spec'd at 250+ MB/s for write. IMO, the write speed issue for SSD's are gone, there's no reason why you shouldn't plunge now.

If you're talking about just read speeds, I think the Corsair SATAIII SSD's have a read of 330 MB/s or something crazy like that, but it drops to 280~MB/s on SATAII, so throughput isn't the only difference.

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