SMR Disk Learning 8---Novel Address Mappings for shingled Write Disks

SMR Disk Learning 8---Novel Address Mappings for shingled Write Disks

Part I: General statement

by changing mappingand reducing The writing amplification problem brought by SWD, this paper mainly discusses two factors, namely SG(Space Gain, spatial gain) and WAR(write amplification Ratio, write magnification) to balance space and performance, and reduce disk system overhead.

Part II: A detailed explanation of the difficulties

Update Out-of-place

I. Analysis of two models of renewal

Update In-place

Update out-of-place requires two region 1. I-region 2.e-region

E-regionUsed as a loop buffer space for caching and re-organizing data. All the input write data arrives firste-regionWhen the data is needed, the data ise-regionTransfer toi-region, that is, in a new place to update, is the original position becomes invalid, this will continue toe-regionand thei-regionthe write operation. Although you can bypassSWDThe problem of writing amplification, but these two Regiongarbage Collection is required (GC) operation reclaims the storage space for invalid blocks whileMapping Tableto make the corresponding changes, so that the system development is larger. This update mode is not a line.

Update In-place does not require GC operations and complex mapping tables,tracks composition Bands, There is a safety interval between adjacent band, which is determined by the width of the head. (Band size is important to weigh space gain and performance)

when the update arrives: the data on the successive tracks that is affected is read into a buffer block where data is rolled back to its original location when the data is updated.

second, to find a breakthrough. Suppose that ordinary disks andSWDsof theTracksorganized in the same way that eachbandinclude4aTracks, traditional disk mappings are continuous, such as[1-100]corresponds to the first Track,[101-200]corresponds to a second Track,[201-300]corresponds to a third Trackand so on. ButSWDIf you use a traditionalMapping, an update may incur additional reads and writes. (One band), updateawill incur2second reading,3Times to write.

Figure 1

If you change the order of tracks to reduce the write amplification problem, you can improve the mapping .

let the capacity utilization of the disk lower than 50% when the band of the first, fourth track , the update generated by the write amplification will be reduced.

So there are a couple of new mapping schemes: R (4123) or R (1423) (mappingthe first25%mapped tobandsthe first Trackon,Mappingthe second25%mapped tobandsthe second Trackon,Mappingof a third25%mapped tobandsof a third Trackon,Mappingthe fourth one25%mapped tobandsthe fourth one Trackon),14R (23)(Mappingthe first50%mapped tobandsthe first to fourth one Trackon,Mappingthe second25%mapped tobandsthe second Trackon,Mappingof a third25%mapped tobandsof a third Trackon),124R (3)(Mappingthe first75%mapped tobandsthe 第一、二、四个 Trackon,Mappingthe second25%mapped tobandsof a third Trackon)

Effect FORECAST:

Third, the experimental verification.

Using four emulated disks and two devices ( address mapper,write amplifier: writing amplifier), respectively, the disk capacity utilization is 25%,50% , 75% , 100% measurement of response time and write amplification ratio for updates

Results

when using the capacity of less than 50% when the write amplification performance is stable,75% performance degradation,100% there is no advantage.

Part III: summary

SWD has write amplification problem, the best way to balance space and performance is new State address addressing: scheme R (4123) or R (1423), 14R (23), 124R (3)

SMR Disk Learning 8---Novel Address Mappings for shingled Write Disks