Media-Card Reader - Dell Precision M7780 Setup And Specifications

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Setup and specifications (34 pages)

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the slowest of the drives. For RAID 0 I/O operations where block sizes are smaller than the stripe size, whichever drive the I/O

operation targets determine the performance, which increases variability and results in inconsistent latencies. This variability is

pronounced for write operations, and it can be problematic for applications that are latency sensitive. One such example of this

is any application that performs thousands of random writes per second in small block sizes.

RAID 1 (Mirrored, Data Protection) volumes benefit from higher performance when drives are matched because the data

is mirrored across multiple drives: all I/O operations must be performed identically to both drives, thus variations in drive

performance when the models are different, results in the I/O operations completing only as fast as the slowest drive. While

this does not suffer the variable latency issue in small random I/O operations as with RAID 0 across heterogeneous drives, the

impact is nonetheless large because the higher performing drive becomes limited in all I/O types. One of the worst examples of

constrained performance here is when using unbuffered I/O. To ensure that writes are fully committed to non-volatile regions of

the RAID volume, unbuffered I/O bypasses cache (for example by using the Force Unit Access bit in the NVMe protocol) and

the I/O operation will not complete until all the drives in the RAID volume have completed the request to commit the data. This

kind of I/O operation completely negates any advantage of a higher performing drive in the volume.

RAID 5 as the most common and best "all-round" RAID level, RAID 5 stripes data blocks across all drives in an array (at least 3

to a maximum of 32), and also distributes parity data across all drives. In the event of a single drive failure, the system reads

the parity data from the working drives to rebuild the data blocks that were lost. RAID 5 read performance is comparable to

that of RAID 0, but there is a penalty for writes since the system must write both the data block and the parity data before the

operation is complete. The RAID parity requires one drive capacity per RAID set, so usable capacity will always be one drive less

than the total number of drives in the configuration. Not suited for applications requiring many small random data writes due to

poor random data write performance.

RAID 10 (sometimes referred to as RAID 1+0) combines RAID 1 and RAID 0 to offer multiple sets of mirrors striped together.

RAID 10 offers good performance with good data protection and no parity calculations. RAID 10 requires a minimum of four

drives, and usable capacity is 50% of available drives. It should be noted, however, that RAID 10 can use more than four drives

in multiples of two. Each mirror in RAID 10 is called a "leg" of the array. A RAID 10 array using, say, eight drives (four "legs," with

four drives as capacity) will offer extreme performance in both spinning media and SSD environments as there are many more

drives splitting the reads and writes into smaller chunks across each drive. Ideal for applications requiring many small random

data writes due to superb random data write performance.

Care must be taken to match not only the drive vendor, capacity, and class, but also the specific model. Drives from the same

vendor, with the same capacity, and even within the same class, can have different performance characteristics for certain

types of I/O operations. Thus, matching by model ensures that the RAID volumes are consisted of a homogeneous array of

drives that deliver all the benefits of a RAID volume without incurring the additional penalties when one or more drives in the

volume are lower performing.

Precision 7780 supports RAID with more than one solid state drive configuration.