Strange RAID Array

[Billy Pilgrim]

Would it be possible to divide an ssd into, maybe 4 or 5 partitions and put them into RAID 5 so if part of the drive failed the data would remain recoverable?

Has this ever been done?

[emosun]

3 minutes ago, Billy Pilgrim said:

if part of the drive failed

it's just not a common failure

and even so..... you can just have more than one copy of each file on the same partition

or buy a regular hard drive and have it back up the ssd every now and then

[Gikero]

The difference between your idea and an actual RAID 5 is that if the single SSD has a failure, all partitions on that SSD are likely lost. Also, they'd be communicating over the same link, thereby limited by that link speed.

[Billy Pilgrim]

18 minutes ago, Gikero said:

The difference between your idea and an actual RAID 5 is that if the single SSD has a failure, all partitions on that SSD are likely lost. Also, they'd be communicating over the same link, thereby limited by that link speed.

 

23 minutes ago, emosun said:

it's just not a common failure

and even so..... you can just have more than one copy of each file on the same partition

or buy a regular hard drive and have it back up the ssd every now and then

Ok, but would it be theoretically possible?

[emosun]

2 minutes ago, Billy Pilgrim said:

Ok, but would it be theoretically possible?

if it's a software raid it could be but it would have almost no advantage and might actually make the ssd slower and fill up faster and not be any more reliable than a single partition.

[Windows7ge]

Like everyone else is saying the SSD itself is a higher point of failure than any one partition on the drive. The benefits would be nil if any. If you want a backup use software to back up the SSD to an HDD. Easier, simpler, and more realistic.

[mariushm]

59 minutes ago, Billy Pilgrim said:

Would it be possible to divide an ssd into, maybe 4 or 5 partitions and put them into RAID 5 so if part of the drive failed the data would remain recoverable?

Has this ever been done?

 

SSDs don't work like mechanical drivers, data is not stored sequentially as clusters and sectors. If you make partitions,that's pointless, because you're not segmenting the data in individual flash memory chips.

 

The SSD controller keeps an internal list of where each chunk of 512 bytes or 4096 bytes (the minimum the drives work with) is stored in flash (which flash memory chip, which layer of memory in the chip because there can be up to 96 memory chips stacked on top of each other to form a single flash chip, which block on that layer, which page in a block). This is needed because flash memory supports a limited number of erases, the flash memory in chips is arranged in portions (blocks) that are typically 512 KB in size which contain pages that are usually 4 KB in size, and these blocks can only be erased (to be rewritten with new data) a limited number of times, in the case of TLC memory approximately 1500-3000 times. So for example, if you edit a text document and change a few letters, the SSD controller can't erase a 4 KB page without erasing a full block of 512 KB, so it will simply mark that 4 KB area as "erasable" and copy the contents with the modifications into a new page in another block which could be in another memory chip (and updates its internal list of sector <-> location in chips)... eventually at some point when the large majority of pages in a block are marked as erasable, only then the 512 KB block is erased and made available for new data.

The SSD controller also hides a portion of the SSD memory to be used as reserves for when such blocks can no longer be written to, and to actually store this list in. That's why you buy 1 TB SSDs and they're formatted to 931 GB, because they make available 1 x 10^12  bytes to the OS  when you actually have approximately 1024 x 2^20 bytes, so around 30-50 GB out of 1 TB of SSD are hidden and kept as reserves. 

 

So even data you already stored, like a bluray movie for example, when the SSD sits idle and you don't access it, it may move data around in memory chips for wear leveling, to minimize the need to do erase in chips.

 

In fact, it also intentionally splits data into as many individual chips as possible when it can, in order to increase read and write speeds - SSD controllers have "channels" similar to DDR memory and it's common for SSD controllers to have 4 or 8 channels, and each channel can have one or more flash memory chips connected to it.

So whenever possible, for example when having to write a big chunk of data, it writes to all 8 channels at the same time, to get higher speeds.

 

edit:  Everyone interested in how SSDs work should read this: Anandtech SSD Anthology, Understanding SSDs: https://www.anandtech.com/show/2738/5

 

[Electronics Wizardy]

1 hour ago, Billy Pilgrim said:

Would it be possible to divide an ssd into, maybe 4 or 5 partitions and put them into RAID 5 so if part of the drive failed the data would remain recoverable?

Has this ever been done?

most ssds already do they, they already have checksums to check for bad bits.

 

And yes you can do this with software raid, but its a dumb idea, let the drive do its things.