Wel­come to the era of ash

ra­di­tional hard dis drives may very well still have a place in mod­ern com­put­ing, but the fu­ture be­longs to !s.

HWM (Singapore) - - Contents - By Kenny Yeo Art Diec­tion by Or­land Pun­za­lan

Six years ago, a 256GB SSD would set you back nearly $600. Today, the same ca­pac­ity drive can be had for as lit­tle as $125 and that same $600 can get you a 1TB SSD with change to spare. Prices of SSDs have fallen dra­mat­i­cally and that has had a pro­found ef­fect on the com­put­ing land­scape.

SSDs are now be­com­ing in­creas­ingly com­mon­place. A re­cent re­port by TrendForce said that the adop­tion rate of SSDs in note­books will hit 50% this year. Put an­other way, one out of ev­ery two new note­books will have an SSD. Due to fall­ing costs, SSDs are also be­com­ing in­creas­ingly pop­u­lar in the en­ter­prise space, es­pe­cially in per­for­mance-crit­i­cal ap­pli­ca­tions. Many re­search rms be­lieve that ship­ments of SSDs will sur­pass HDDs in the next two to three years. So what is it about SSDs that gives them a leg up over its me­chan­i­cal ri­vals? Let’s look at some key con­sid­er­a­tions now.

HDD VS. SSD Per­for­mance

We all know per­for­mance is one of the big­gest ad­van­tages that SSDs have over HDDs. Even the most ba­sic SSD will run cir­cles around the fastest, most ad­vanced HDDs. SSDs are there­fore well suited to per­for­mance-crit­i­cal ap­pli­ca­tions or tasks where high per­for­mance and through­put is re­quired. For con­sumers, SSDs pro­vide signicantly shorter boot­ing up and load­ing times. In the en­ter­prise space, SSDs are suit­able for data an­a­lyt­ics, vir­tu­al­iza­tion, and cloud ser­vices. rvices.

To give you an idea of justst how fast SSDs are, con­sider the tablee op­po­site page:

Ca­pac­ity

HDDs used to have the edge over SSDs be­cause of their higher ca­pac­i­ties. But this is no longer the case. Sam­sung an­nounced a 15TB SSD in 2016 and fol­lowed up with a 30TB SSD just ear­lier this year that comes in a slightly thicker 2.5-inch form fac­tor. For busi­nesses who need more, Nim­bus Data’s Ex­aDrive DC100 is a 100TB SSD that comes in stan­dard 3.5-inch form fac­tor. In com­par­i­son, the largest ca­pac­ity HDD cur­rently holds only 14TB of data.

Power

SSDs are also con­sid­er­ably moree power r e cient than HDDs. For mo­bile de­vices­de­vices, , us­ing SSDs in place of HDDs can ex­tend d bat­tery life. In the en­ter­prise space,ce, this cru­cially means they gen­er­ate lessss heat so less ro­bust cool­ing is re­quired,, which h trans­lates to eas­ier main­te­nance and lower run­ning costs.

Shock and vi­bra­tion

SSDs have no mov­ing parts, which makes them signicantly more re­sis­tant to shocks as com­pared to HDDs. HDDs can be eas­ily dam­aged if they are jolted when in use. And since SSDs have no mov­ing parts, they don’t vi­brate. This makes them eas­ier to de­ploy in data cen­ters. On the other hand, su cient care must be taken to en­sure that data racks and servers have the struc­tural rigid­ity re­quired to with­stand the vi­bra­tion caused by mul­ti­ple HDDs spin­ning si­mul­ta­ne­ously.

Re­li­a­bil­ity

SSDs have a life­span. There are only so many times you can write to a cell. How­ever, SSD en­durance and re­li­a­bil­ity have been steadily im­prov­ing and are now as good as, if not bet­ter, than HDDs. One way of look­ing at re­li­a­bil­ity is MTBF or mean time be­tween fail­ures. En­ter­prise-grade SSDs have an MTBF of 2 to 2.5 mil­lion hours, sim­i­lar to en­ter­priseg­rade HDDs.

For con­sumers, a bet­ter way of look­ing at re­li­a­bil­ity is en­durance. Take, for ex­am­ple, a 512GB ver­sion of Sam­sung’s new SSD 860 Pro. That has a rated en­durance of 600TBW, which means you can write 600TB of data to it be­fore it fails. In most client us­age sce­nar­ios, most users will write, at most, about 30GB of data to their drives a day. Even if you are a heavy user and write 50GB of data per day, 600TBW is enough for roughly 32 years.

Cost

Even though prices of SSDs have fallen signicantly. They are still con­sid­er­ably more ex­pen­sive than HDDs. Sam­sung’s 15TB en­ter­prise-grade PM1633a SSD, for ex­am­ple, re­tails for over US$10,000. On the other hand, a 12TB en­ter­prise-grade HDD typ­i­cally costs about US$500. Things are not much bet­ter on the client side. A 1TB SSD will set you back S$500, but a 1TB HDD can be had for as lit­tle as S$70.

THE AR­RIVAL OF 3D NAND

Though the rst SSD was shipped in 1991, SSDs in the client space only ar­rived about ten years ago. Back then, it was widely be­lieved that SSDs could never com­pete against HDD when it came to ca­pac­ity and re­li­a­bil­ity. The rea­son for that was be­cause of the lim­i­ta­tions of tra­di­tional NAND ar­chi­tec­ture.

In the past, ash mem­ory man­u­fac­tur­ers sought to in­crease mem­ory den­sity, and hence ca­pac­ity, by ei­ther us­ing smaller man­u­fac­tur­ing pro­cesses to cram more cells onto the die or by in­creas­ing the num­ber of bits

that can be stored per mem­ory cell. Both ap­proaches have their prob­lems.

There is a limit to how much cells one can squeeze onto the die. Since each cell has to hold a charge, squeez­ing too many cells onto the die re­sults in less space be­tween each cell. In­ter­fer­ence from neigh­bor­ing cells can dis­rupt or even in­ad­ver­tently change the state of the cell that it is next to and cause the data to be­come cor­rupt.

On the other hand, one can keep the size of cells con­stant but in­crease the amount of data stored on it. There are, how­ever, two key down­sides to this ap­proach and they are per­for­mance and en­durance. By in­creas­ing the num­ber of bits per cell, you are in­creas­ing the num­ber of volt­age states for the drive con­troller to check. This re­duces drive per­for­mance.

Ad­di­tion­ally, by hav­ing more stages in each cell, the mar­gin be­tween in­di­vid­ual stages is re­duced. And since writ­ing to the cell grad­u­ally re­duces the layer of sil­i­con ox­ide within that acts as an in­su­la­tor be­tween the var­i­ous stages, this ef­fec­tively means that each cell has a shorter life­span.

These are the prob­lems fac­ing tra­di­tional NAND ar­chi­tec­ture. And in 2014, Sam­sung made a break­through with what they call 3D NAND or V-NAND (Ver­ti­cal NAND), and this has gone on to be­come one of the most im­por­tant in­no­va­tions in the ash mem­ory busi­ness.

Sam­sung’s so­lu­tion to the prob­lems fac­ing NAND is el­e­gant and ef­fec­tive. As its name sug­gests, 3D NAND over­comes the lim­i­ta­tion of tra­di­tional two-di­men­sional pla­nar NAND by stacking cells upon each other. This not only al­lows more cells to be placed on a sin­gle die, it also cre­ates more space be­tween cells, al­low­ing Sam­sung to roll back to a less ag­gres­sive man­u­fac­tur­ing process node, thereby im­prov­ing per­for­mance and en­durance.

Sam­sung’s rst 3D NAND SSD to go into pro­duc­tion was the SSD 850 Pro from 2014 and its 3D NAND mem­ory con­sisted of 24 stacked lay­ers of cells. As of now, Sam­sung has al­ready put into pro­duc­tion 3D NAND with 64 lay­ers of cells. This has al­lowed them to cram 4TB into a stan­dard 2.5-inch form fac­tor and 2TB on a stan­dard M.2 2280 mem­ory drive. And at the Flash Mem­ory Sum­mit last year, Sam­sung has shared that they are al­ready work­ing on 3D NAND with 96 lay­ers of cells. Today, the vast ma­jor­ity of SSDs em­ploy 3D NAND, which is a tes­ta­ment to the im­por­tance of this NAND tech­nol­ogy.

Be­yond 3D NAND, ash mem­ory man­u­fac­tur­ers are al­ready ex­plor­ing dif­fer­ent types of mem­ory. In­tel and Mi­cron have a new type of mem­ory called 3D XPoint, which of­fers sig­nif­i­cantly greater per­for­mance and en­durance than NAND mem­ory. It is mar­keted to ad­dress the per­for­mance gap be­tween NAND and DRAM.

Not to be out­done, Sam­sung is also work­ing on a new type of mem­ory that they call Z-NAND. Like 3D XPoint, Z-NAND is a high per­for­mance, high en­durance mem­ory that slots be­tween NAND and DRAM. And like In­tel and Mi­cron, Sam­sung has not shared de­tails about this new mem­ory, but the rst drive to use Z-NAND, the en­ter­prise-fo­cused SZ985, looks mighty promis­ing.

3D NAND or V-NAND (Ver­ti­cal NAND) has gone on to be­come one of the most im­por­tant in­no­va­tions in the ash mem­ory busi­ness.

3D NAND STRUC­TURE

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