SD cards – postage stamp sized flash cards in your camera – have no internal cache, little internal bandwidth, tiny CPUs, and slow I/O busses. But recent tests found that SD cards could be 200 times faster than an SSD. How???
Recently the good folks at Datalight published results of testing they’d done on their Reliance Nitro transactional file system and Microsoft’s Texfat (Transaction-Safe Extended FAT). Both are used in embedded systems like ATMs and mobile devices with critical data reliability needs.
HOW CAN SLOWER BE FASTER?
There’s no doubt that SSDs have more raw performance than SD cards. But in this case its not what is used, but how it is used.
Datalight found that creating a file and writing a file with Texfat on an SSD took over 8 seconds, while the same operations on an SD card took anywhere from 41 to 101ms.
When they investigated, they found that the SSD
. . . is actually doing something (flushing data out of a cache internal to the SSD, most likely), whereas on SD – it does no additional work (since SD has no internal cache), and so . . . SD appears “faster”.
If you worry about corrupting data due to power loss, those 8 seconds can be an eternity.
As Datalight points out:
. . . the path taken by data from application to media can be convoluted, passing through caches and buffers at different layers of the system in an effort to improve or maintain performance.”
While the caches and buffers are an architectural strategy to improve performance, Datalight’s testing shows that they don’t always do so.
THE STORAGE BITS TAKE
No, you shouldn’t replace your SSDs with SD cards, though some are sporting faster-than-disk specs.
Rather, I see Datalight’s results as evidence that the old disk-oriented software stacks and file systems with many layers of caching are less and less relevant in a world of high-performance solid state storage. For 50 years all systems were engineered to minimize the long latency and low-IOPS of disks – and with fast SSDs much of that careful engineering is obsolete and counter productive.
If you are engineering embedded systems, the Datalight blog post is worth a close read. If you are running data systems, you should be more aggressive in understanding how your chosen software works – or not – with SSDs.
SD kadi <unk> postage stempu ukubwa flash kadi katika kamera yako <unk> hawana cache ya ndani, kidogo ya ndani bandwidth, CPUs ndogo, na polepole I / O mabasi. Kwa mfano, katika majaribio ya hivi karibuni, kadi za SD zilikuwa na uwezo wa kuendesha kwa kasi mara 200 kuliko SSD. Jinsi gani??? Hivi karibuni watu wazuri katika Datalight kuchapishwa matokeo ya majaribio walikuwa wamefanya juu ya Reliance Nitro yao transactional faili mfumo na Microsoft ya Texfat (Transaction-Safe Extended FAT). Wote hutumiwa katika mifumo iliyoingizwa kama ATMs na vifaa vya rununu na mahitaji muhimu ya kuegemea data. Jinsi ya Kuchukua Hatua Polepole? Hakuna shaka kwamba SSDs na utendaji zaidi ghafi kuliko kadi SD. Lakini katika kesi hii si nini hutumiwa, lakini jinsi inatumiwa. Datalight iligundua kuwa kuunda faili na kuandika faili na Texfat kwenye SSD ilichukua zaidi ya sekunde nane, wakati shughuli sawa kwenye kadi ya SD ilichukua mahali popote kutoka 41 hadi 101ms. Baada ya uchunguzi, waligundua kwamba SSD ilikuwa na . . "Kama ""kuondoa data kutoka cache ya ndani ya SSD"" (kwa kweli, ""kuondoa data kutoka cache ya ndani ya SSD"), wakati kwenye SD <unk> haifanyi kazi ya ziada (kwa kuwa SD haina cache ya ndani), na hivyo." . . SD inaonekana <unk>haraka<unk>. Ikiwa una wasiwasi juu ya data ya uharibifu kutokana na upotezaji wa nguvu, sekunde hizo nane zinaweza kuwa umilele. Kama Datalight inaonyesha:. . . Njia inayotumiwa na data kutoka kwa programu hadi vyombo vya habari inaweza kuwa ngumu, ikipita kupitia caches na buffers katika tabaka tofauti za mfumo katika juhudi za kuboresha au kudumisha utendaji. <unk> Wakati caches na buffers ni mkakati wa usanifu ili kuboresha utendaji, upimaji wa Datalight unaonyesha kwamba hawafanyi hivyo kila wakati. STORAGE BITS TAKE Hapana, unapaswa <unk>t kubadilisha SSD yako na kadi SD, ingawa baadhi ni michezo kwa kasi-kuna-diski maelezo. Badala yake, naona matokeo ya Datalight kama ushahidi kwamba zamani diski-oriented programu stacks na mifumo ya faili na tabaka nyingi za caching ni chini na chini ya muhimu katika ulimwengu wa juu-utendaji imara hali ya kuhifadhi. Kwa miaka 50 mifumo yote walikuwa engineered kupunguza latency muda mrefu na chini-IOPS ya disks <unk> na na SSDs haraka mengi ya uhandisi kwamba makini ni ya kizamani na counter-matokeo. Kama wewe ni uhandisi mifumo iliyoingizwa, Datalight blog post ni thamani ya kusoma karibu. Kama wewe ni kuendesha mifumo ya data, unapaswa kuwa na nguvu zaidi katika kuelewa jinsi programu yako iliyochaguliwa kazi <unk> au si <unk> na SSDs.