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Comment Re:Terabyte flash drives are 10% overprovisioned (Score 1) 264

I wasn't actually thinking of DoS'ing, but I guess that's actually a valid concern. If a particular write pattern could crap a server, then you may have to worry about a user doing that to your server. I was just putting my "DV engineer" hat on, and trying to think of how I'd break an SSD in the minimum number of writes. It's the kind of analysis I'd hope the engineers that come up with lifetime specs use to give a bulletproof lifetime spec. For example, X years at YY MB/day even if you're writing like an a**hole. ;-)

I don't have a formalized attack against any particular drive, manufacturer or filesystem.

For a multi-user system, just a thought: Could you address it with quotas? If a given user can't write to more than X% of the filesystem, you can bound the "badness" of their behavior.

Comment Re:Oh goody (Score 1) 264

I'm not challenging the 30 day number, to be sure.

It's not entirely true that write amplification won't appear to speed up the rate at which an SSD erases sectors. SSDs generally have multiple independent flash banks, and each can process an erasure independent of the others. To maximize your erasure rate, you need a pattern of writes that triggers erasures across all banks as often as possible. Each bank will split its time spent receiving data to write, committing write data to flash cells, and erasing flash cells. (My assumption is that a given bank can only be doing one of these operations at a time, which was certainly true for the flash devices I programmed.)

Consider a host sending a stream of writes as fast as it can send it. The writes will land on the drive as fast as the SSD controller can process them and direct them to flash cells. If there are any bottlenecks in that path, such as generating ECC codes and allocating physical blocks in the FTL, it will slow down the part of the duty cycle devoted receiving and committing write data.

A "friendly" write stream would minimize the number of GC cycles the SSD performs, and thus the amount of write amplification that occurs. Thus, the total number of writes to the SSD media is at most slightly larger than what the PC sends, and the "receive-write" portion of the "receive-write-erase" cycle gets lengthened by whatever bottlenecks might be in the PC-controller-flash path. A "hostile" write stream triggers a larger number of GC cycles to migrate sectors. It seems reasonable to me that an on-board chip-to-chip block migration might be quite a bit faster than receiving data from the PC. For one thing, you don't necessarily need to recompute ECC. The block transfer itself could be handled by a dedicated DMA-like controller transferring between independent banks in parallel with other activity. So, generating more write data locally to the SSD could reduce the time spent in the receive-write portion of the receive-write-erase cycle, so you can spend a greater percentage of your time erasing as opposed to receiving or writing.

It seems a little counter-intuitive, but it's in some ways similar to getting a super-linear speedup on an SMP system, which is indeed possible with the right workload. How? By keeping more of the traffic local.

The main effect of write amplification, though, is on the SSD wear specs themselves, as I said. They're stated in terms of days/months/years of writes at a particular average write rate. So really, when you multiply that out, they're specified in terms of total writes from the PC. There's at least one flash endurance experiment out there showing that drives often massively exceed their rated maximum total writes by very large factors. One reason for that, I suspect, is that they aren't sending challenging enough write patterns to the drive to trigger worst case (in terms of bytes written, not wall-clock time) failure rates.

Comment Re:Terabyte flash drives are 10% overprovisioned (Score 1) 264

OK, I see that SandForce has on-the-fly compression tech, which I imagine would help more reasonable workloads. (Although, if your workload involves a lot of compressed video or images, that compression tech won't buy you anything.)

The point of my thought experiment, though, was how I would construct a maximally bad workload, and it's pretty easy to nullify compression with uncompressable data.

Comment Re:Terabyte flash drives are 10% overprovisioned (Score 1) 264

A 4TB drive would bother with compression why exactly? It won't improve any benchmarks. Someone like me trying to break the media would just write uncompressible garbage anyway.

A large overprovisioning pool does help stay in the dynamic wear leveling paradigm longer. If the drive performs any amount of static wear leveling, though, where it can get the rest of the sectors into the fun and there's a limited aging ratio / aging difference between the most-erased sector and least-erased sector, then the size of the overprovisioning space doesn't matter too much this attack—rather, the total media size matters.

The point of this attack is to limit the ability of the SSD to separate disk sectors into hot and cold effectively, so you can force the maximum number of erasures through garbage collection cycles. As long as the overprovisioned space is less than the ratio of exposed sector size (512 or 4K bytes) to erase sector size (likely something huge like 512K bytes), you can force a lot of erasures just for GC compaction. The drive will still spread these erasures out as much as possible. With the blended dynamic/static wear leveling model, you'll end up aging all of the sectors roughly evenly.

Once you get one sector to fail and get taken out of service, you're close to making as many more fail as you like, since the SSD did everything it could to age sectors evenly. The additional delta work you need to get it to fail completely isn't very big.

In case my GC point wasn't clear: Consider a simple SSD with a capacity of 32 "blocks", of which it advertises a capacity of 24 to the user, leaving 8 for overprovisioning. Suppose that filesystem blocks get grouped into erase blocks with a 4:1 ratio. Now suppose I've filled that disk up to capacity with a linear series of writes. You might represent it like so, with letters representing FS blocks with data, dashes representing empty clean FS blocks that the FTL can write to, and the groups of 4 separated by dots representing the erase blocks:

abcd.efgh.ijkl.mnop.qrst.uvwx.----.----

Now suppose I write to A, E, I, M. I need to migrate these FS blocks to new locations to absorb the writes. Their old locations are freed, but remain dirty (cannot be rewritten). After these 4 writes, my flash might look like this: (The '#' indicate free-but-dirty FS blocks.)

#bcd.#fgh.#jkl.#nop.qrst.uvwx.aeim.----

Before this SSD can execute my next rewrite, the flash needs to perform a GC cycle to ensure it always has a place to migrate data for GC. So, before processing another rewrite, it performs a GC cycle, migrating blocks until it has at least clean two erase sectors. For the sake of argument, let's assume it employs a simple round robin scheme to spread the GC over the media evenly:

##cd.#fgh.#jkl.#nop.qrst.uvwx.aeim.b--- Migrate 'b'.
###d.#fgh.#jkl.#nop.qrst.uvwx.aeim.bc-- Migrate 'c'.
####.#fgh.#jkl.#nop.qrst.uvwx.aeim.bcd- Migrate 'd'.
----.#fgh.#jkl.#nop.qrst.uvwx.aeim.bcd- Erase first sector.
gh--.####.#jkl.#nop.qrst.uvwx.aeim.bcdf Migrate 'f', 'g', and 'h'.
gh--.----.#jkl.#nop.qrst.uvwx.aeim.bcdf Erase second sector.
ghjk.l---.####.#nop.qrst.uvwx.aeim.bcdf Migrate 'j', 'k', 'l'.
ghjk.l---.----.#nop.qrst.uvwx.aeim.bcdf Erase third sector.
ghjk.lnop.----.####.qrst.uvwx.aeim.bcdf Migrate 'n', 'o', 'p'.
ghjk.lnop.----.----.qrst.uvwx.aeim.bcdf Erase fourth sector.

Now I continue with my dickish attack, and rewrite Q, U, A, and B:

#hjk.#nop.quab.----.#rst.#vwx.#eim.#cdf

Oh, hey, that'll trigger another GC cycle to free up a sector:

#hjk.#nop.quab.rst-.####.#vwx.#eim.#cdf Migrate 'r', 's', 't'
#hjk.#nop.quab.rst-.----.#vwx.#eim.#cdf Erase fifth sector
#hjk.#nop.quab.rstv.wx--.####.#eim.#cdf Migrate 'v', 'w', 'x'
#hjk.#nop.quab.rstv.wx--.----.#eim.#cdf Erase sixth sector
#hjk.#nop.quab.rstv.wxei.m---.####.#cdf Migrate 'e', 'i', 'm'
#hjk.#nop.quab.rstv.wxei.m---.----.#cdf Erase seventh sector
#hjk.#nop.quab.rstv.wxei.mcdf.----.#### Migrate 'c', 'd', 'f'
#hjk.#nop.quab.rstv.wxei.mcdf.----.---- Erase eighth sector

Keep up the pattern, writing to the first FS block in each erase block, and you'll maximize the number of erasures due to GC. Sure, all those erasures take time, but you can bring your time to failure down to a function of the total number of erasures the media can tolerate before it fails in a minimal number of writes. If the media has any parallelism between flash modules (so that it can execute multiple erasures in parallel), you may even be able to get that working in your favor, assuming your explicit goal is to make the drive fail as quickly as possible.

Comment Re:Oh goody (Score 1) 264

The point of write amplification is solely to get more sectors into the erasure party. A single small write forces the SSD to migrate full sector A to empty sector B.

If you could send direct physical sector erasure and physical write commands to the media, you could just tell it to erase each sector and rewrite its first byte repeatedly until that sector failed, and then march to the next sector.

But, you don't have the opportunity to do that. Instead, you must interact at the filesystem level, and there's an FTL between the file system and the media. So, if your goal is to ruin the media quickly with those two layers between you, you want to minimize the FTL's ability to filter writes out and reduce the required number of erasures.

I'm aware that erase times for sectors are huge, and will slow your I/O rate accordingly. You're right that write amplification doesn't necessary shorten the calendar days to failure, as a different write pattern may have triggered the same number of erasures in the time frame with a larger number of writes. But, writes aren't free either, so there's at least some, err, benefit to minimizing the number of writes required to ruin your SSD, if your goal is to ruin your SSD as quickly as possible.

Comment Re:Oh goody (Score 1) 264

If you want to write once and forget it, you can fill the thing right to the top of the advertised capacity, and you don't have to worry about failure due to wear. Instead, you have to worry about failure due to electrons migrating off the bits. So, you need to refresh all the bits every so often, much like DRAM, only with a much slower refresh interval. Even if you refresh all the bits on the drive once a day, if you do so in a nice, orderly manner, I'd imagine you won't reach the rewrite limit for the drive in your lifetime.

Still, I'm not sure I'd choose an SSD for that.

Comment Re:Oh goody (Score 1) 264

Yes, I know what Stacker, DoubleSpace and DriveSpace were as a technical implementation. My point is mainly that modern Windows still offers a mechanism to compress files on a live filesystem. It just lets you select at folder granularity rather than whole-disk granularity. The fact that they're not implemented at the same layer of the stack I didn't think was relevant here.

Stacker et al worked at the sector level so that you didn't need to modify DOS or the umpteen programs that made use of sector-level access to the filesystem, and insisted on that level of access in order to function. Copy protection schemes and disk editors both relied on it. (Defraggers too, although defragging a compressed volume is... crazy.) Databases may have as well; I'm not certain. Windows NT forces programs through a narrower, more controlled window of APIs to access the file system.

I actually had a Stacker'd hard drive back in the day and had read up on all the tech, so it's not like I'm unfamiliar with it.

Comment Re:Oh goody (Score 1) 264

Ok, I just checked in my WinXP box: You can right click on a folder, go to "Properties". Click "Advanced", and there's an option to "Compress to save disk space." I'm too lazy to go get my Win7 laptop to see if that's still there.

So, some version of TroubleSpace...err...DoubleSpace...err...DriveSpace survived beyond Win98.

Comment Re:Oh goody (Score 3, Informative) 264

If you know something about the drive's sector migration policies, in theory you could construct a worst-case amplification attack against a given drive. Leverage that against the drive's wear leveling policies. But, that seems rather unlikely.

Flash pages retain their data until they're erased. You can write at the byte level, but you must erase at the full page level. You can't rewrite a byte until you erase the page that contains it. That's the heart of the attack: Rewriting sectors with new data. You can't rewrite a sector in-place. You mark the old location as "dirty but free", and write the new data to a new location. The SSD can't reclaim the dirty-but-free sectors for writing until they're erased.

Thus, the basic idea goes something like this: Fill the disk to 99.9% full. Then, selectively rewrite individual sectors, forcing the sector to migrate to a new flash page. Wash, rinse, repeat until the drive fails.

If the drive only performs dynamic wear leveling, all subsequent rewrites will erase and reuse only among the free space. (Note: This free space includes all of the space the drive reserves to itself for dynamic wear leveling purposes.) Now all you need to do is reach the erase/rewrite limit among the available dynamic wear leveling pool, which is significantly smaller than the full drive capacity. You can achieve this by rewriting a small subset of sectors until the disk falls over.

Modern drives perform a blend of dynamic and static wear leveling. Dynamic wear leveling only erases/rewrites among the "free" space. Static wear leveling gets otherwise untouched sectors into the fray by wear leveling over all sectors. This blended approach defers static wear leveling until it becomes absolutely necessary. The flash translation layer (FTL) detects when the wear difference between sectors gets too imbalanced, and migrates static sectors into the worn regions and wear-levels over the previously "static" sectors.

A successful attack would take this into account and attempt to keep track of which sectors would be marked "static" vs. "dynamic". It would also predict how the static sectors were grouped together into pages, so it could cherry-pick and inflict the maximum damage: All it needs to do is write to a single sector in each static flash page (creating a bunch of unallocated "dirty-but-free" holes), continuing until the SSD was forced into a garbage collection cycle. That GC cycle then would have to touch all the static pages (or at least a significant fraction) to compact the holes away and make space available for future writes.

If you can keep that up, you can magnify your writes by the ratio between the page size and the sector size. If you have 512 byte sectors and 512K bytes pages, the amplification factor is 1024.

But, as I suggested above, to achieve this directly, you need to have some idea of how the SSD marks things static vs. dynamic. Without such knowledge, you have to approximate.

I imagine if you really wanted to kill an SSD without any knowledge of its algorithms, you could do something simple like rewrite every allocated sector in an arbitrary order, shuffling the order each time. SSD algorithms assume a distribution of "hotness" (ie. some sectors are "hot" and will be rewritten regularly, and most are "cold" and will be rewritten rarely if ever), and so rewriting all sectors in a random order will cause rather persistent fragmentation, recurring GC cycles, and pretty noticeable amplification.

You wouldn't get to the 40 day mark, but if you started with a mostly full SSD, you might get to a few months.

That's my back-of-the-napkin, "I wrote an FTL once and had to reason through all this" estimate.

Comment Re:It's great, but we try not to use it. (Score 1) 435

I actually use C++ for embedded programming, because when used with care, it can actually do a better job than C for a number of things. I use template meta-programming to compute various things at compile time, such as, say, register initialization values and what not. Sure, I can do the same with #define and a boat load of macros, but that has its own issues. Not only are macros messy in their own way, they don't provide a good way to sanity check your settings. With templates and types done right, I can actually get the compiler to sanity check my settings at compile time. I don't know how many times I've chased down a bug due to swapped macro parameters that could have been caught compile-time with some type checking / trait checking.

I've written an entire C++ based support library just for this purpose. One of its goals is extreme compactness and cycle efficiency, since the code often needs to run in RTL simulation. Software RTL simulation of a large SoC runs in the 10s to 1000s of cycles per second, so cycle efficiency is at an extreme premium.

What my library largely replaces is other C and assembly code that (often hamfistedly) computes everything at run time, and so my code can handily beat that.

I haven't quite hit the nirvana of generating an entire MMU page tree from a compact memory map description using templates (I have a perl script for that), but it sure beats 100,000s cycles or more computing it at run time when that translates to hours of sim time. (Fun fact: Some rather popular modern processors run really slow until you turn the MMU on, because they can't cache any data until you do.)

I have however written dynamic code generators that use templates and function overloading to resolve as much of the opcode encoding as possible at compile time, so that the run-time portion usually is just a "store constant" or maybe a quick field insert into a constant followed by a store. Those can pump opcodes to memory as fast as an opcode per cycle (and in some special cases, faster), which is pretty darn good. Again, all typechecked as much as possible at compile time, to minimize or eliminate the possibility I generate invalid instructions.

Comment Re:STL is painful to use (Score 1) 435

Suppose you want to determine if a collection c contains an element e. In any other language, you'd write something like c.contains(e).

I have good news for you! Sure, you still need to provide begin() and end() to specify a range, but it's a step forward. And, with the new non-member begin() and end() you can even use it on plain arrays.

Yeah, you still have to put all the pieces together yourself, but the pieces are a bit more uniform now and there's usually fewer to worry about. (Especially now with auto.)

Comment Re:C++ admits it is too complex with "auto" for ty (Score 1) 435

Before auto it seemed like C++'s error messages were downright passive aggressive: "If you don't know what to put here, I'm not going to tell you." At least, it's not going to tell me the concise thing to put there. It will tell me the completely flattened type, which can be quite huge if you're trying to, say, get an iterator to a nested STL container holding a template class composed against some other classes (that themselves might be templated) a'la the Policy pattern.

I just wish I didn't have to code for the lowest common denominator compiler at work, so I can be sure I can use auto with impunity. :-)

Comment Re:Simple (Score 1) 435

I unfortunately claim ignorance on the license for the runtime. I know some of my employer's products use Dinkumware for the C++ library, but I'm not sure what this processor uses. (TMS320C6600 family, if you're curious.) I'm usually at the other end of the pipeline, using the pre-alpha tools before the silicon exists, so I'm pretty far removed from the customer toolchain distribution end of things. Sorry I can't be more helpful on that detail. I can tell you all about VLIW instruction scheduling and cache memory system pipeline behavior though!

Comment Re:The STL is too general purpose (Score 1) 435

You do have to be sure to compile with full optimization enabled, though, for STL to have a minimal hit. I use STL quite happily to do things I only too eagerly rolled my own implementation of years ago, and then clung to, even if it wasn't a perfect fit. For example, for eons I carried around this AVL tree implementation I wrote for a data structures class, and used it to implement associative containers, just so I wouldn't have to do it again. These days, it's simply map< yadda, yadda > and I'm on my way. I'm willing to bet map<> beats that creaky old AVL tree any day.

Without optimization, the STL containers can slow down quite a bit. I've heard the effect is especially large on some versions of MSVC++, since they have special debugging versions of the iterators that incur their own cost penalties in return for other checks. I wouldn't know; I do all my development under Linux or for embedded processors on bare metal.

With optimization on, I rarely if ever notice a performance issue due to STL. I do run into the occasional limitation, such as needing an actual resizeable 2-D array-like structure. (A vector< vector< ... > > doesn't cut it, because resizing the inner dimension doesn't resize all rows.) But, that's more exception than rule.

My biggest complaint about C++11 is that I won't realistically be able to use it for another few years. Grrr.

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