4 ms·

Yes, that's true though "blocks" is an overloaded term (e.g., people will talk about what "block size" they are reading at at the application layer). In any ca

Yes, that's true though "blocks" is an overloaded term (e.g., people will talk about what "block size" they are reading at at the application layer). In any case I'm mostly disputing your characterization of SSDs as behaving like hard drives: having long setup time then faster reading of subsequent blocks. Unless you are talking about command latency, they don't really work like that. To a first order approximation you can think of SSDs a ideal page-wise parallel random access devices, with a certain read latency and the ability to be processing up to N reads at once. As long as the queue depth can keep up to N reads in progress at once, the maximum bandwidth or a substantial fraction, can be achieved.

sitkack 4y ago
We are definitely in the weeds. When used via an OS, you get better performance when you treat an SSD like a fast tape than you do if you do random IO (in the application domain, in the SSD domain at a high enough granularity yes, they are random access). There is setup time in opening a page, writing blocks within the page, sealing the page. If your writes are less than an SSD block, it has to copy the existing data to a buffer, write it, append the new data, seal the block. Or it applies a translation layer. SSDs are complex devices and are not fundamentally random access, no more so than DRAM, which is also not random access.
- BeeOnRope 4y ago
  You were talking about reads and I responded about reads. I'll repeat my claim: SSDs largely behave as random access devices for page-aligned random reads of an integer multiple of pages. In particular, without any performance footguns activated in the application & OS stack, you can get a substantial portion or all the throughput with such a random read workload compared to the most sequential read workload you can imagine. This is totally unlike disks (resp. tapes) where you have the hard physical reality of average seek time in the milliseconds (resp. seconds) making random read loads orders of magnitude slower (resp. even more orders) than sequential ones. Writes are different and more complicated, but they also behave as random write devices if you choose your granularity to be "block size" and even at page size (if you can avoid write amplification, which is not always possible). You have page and block reversed in your description: a page is smaller than and contained within a block. SSDs to do not "open a block, overwrite a page and then seal the block": they write newly written pages to a freshly erased block, using a variety of possible strategies, which sometimes give an advantage to nearby-in-time writes to the same block or sometimes not.

sitkack 4y ago

We are definitely in the weeds. When used via an OS, you get better performance when you treat an SSD like a fast tape than you do if you do random IO (in the application domain, in the SSD domain at a high enough granularity yes, they are random access). There is setup time in opening a page, writing blocks within the page, sealing the page. If your writes are less than an SSD block, it has to copy the existing data to a buffer, write it, append the new data, seal the block. Or it applies a translation layer. SSDs are complex devices and are not fundamentally random access, no more so than DRAM, which is also not random access.

BeeOnRope 4y ago
You were talking about reads and I responded about reads. I'll repeat my claim: SSDs largely behave as random access devices for page-aligned random reads of an integer multiple of pages. In particular, without any performance footguns activated in the application & OS stack, you can get a substantial portion or all the throughput with such a random read workload compared to the most sequential read workload you can imagine. This is totally unlike disks (resp. tapes) where you have the hard physical reality of average seek time in the milliseconds (resp. seconds) making random read loads orders of magnitude slower (resp. even more orders) than sequential ones. Writes are different and more complicated, but they also behave as random write devices if you choose your granularity to be "block size" and even at page size (if you can avoid write amplification, which is not always possible). You have page and block reversed in your description: a page is smaller than and contained within a block. SSDs to do not "open a block, overwrite a page and then seal the block": they write newly written pages to a freshly erased block, using a variety of possible strategies, which sometimes give an advantage to nearby-in-time writes to the same block or sometimes not.