Getting The Most Out Of Dd: Tweaking Block Size For Maximum Drive Performance

What is Block Size and Why It Matters

The block size in dd refers to the chunk of data, measured in bytes, that is written to or read from the input/output drive during each transfer operation. Choosing an optimal block size for your specific drive and use case can significantly impact the write/read speeds and overall drive performance.

Writing data in large contiguous blocks minimizes drive seeks and generally achieves much faster throughput compared to small scattered writes. However, if the block size is set too large, it can exhaust system memory and buffers resulting in slower speeds. Finding the right balance is key.

Certain use cases, such as writing multiple small files rather than large files, also favor smaller block sizes. We dive deeper into the factors that determine the ideal block size and recommend optimally tuned dd settings for common drive types and operations.

Table of Contents

Definition of block size in dd

The block size in dd is defined in bytes and specifies the chunk of data transferred during low-level read/write operations to the output drive. The default block size in dd is 512 bytes but values can range from 1 byte to entire megabytes/gigabytes.

Each block write/read involves additional overhead – updating metadata, drive seeks etc. Larger block sizes minimize this overhead by transferring more sequential data in each operation.

Impact of block size on drive write speeds

The dd block size significantly impacts how fast data can be written to or read from a drive. An optimally sized block can come close to saturating the maximum write bandwidth of HDDs, SDDs and other high speed devices.

In general, larger block sizes result in faster peak transfer rates. 4K to 128K seems widely recommended for HDDs whereas SSDs see benefits beyond 512K. NVMe drives with phenomenal bandwidth can utilize block sizes in MBs.

However, excessively large blocks may throttle performance – 1MB block on a hard disk is likely too much. There is a memory and buffer sweet spot beyond which returns diminish. Proper tuning to find this sweet spot can boost dd speeds by up to 10x.

When smaller is faster – use cases

While sequential throughput sees benefits from larger block sizes, certain use cases favor smaller blocks. Writing multiple smaller files rather than a single large file is one such example. Here multiple smaller writes would be faster than fewer larger ones.

Operations involving random access rather than purely sequential i/o also tend to perform better with smaller block sizes. SSDs in particular can have faster random iops with moderate block sizes. Hard disk drives benefit from aligned smaller blocks in random use.

When cloning a drive containing multiple partitions and operating systems, smaller block sizes help efficiently replicate partition scheme along with data. Granularity can be useful even when dealing with single large files occasionally.

Finding the Sweet Spot

Factors that influence best block size

Theideal or sweet spot block size depends on several aspects – the type of device (HDD vs SDD vs NVME), interface/bus bandwidth, average file sizes, operating system’s memory limits, natural IO sizes etc.

Fundamentally there is a tradeoff between minimizing transfer overhead with larger blocks and minimizing fragmentation/idle waits with smaller writes.
Modern operating systems already try optimizing default IO block sizes but still allow tuning write workloads.

On traditional HDDs, going beyond 128KB block size hits a point of diminishing returns and 4-64KB is reasonable. SATA SSDs achieve great performance at higher blocks from 512KB to 1MB. NVMe drives have the bandwidth for even larger block sizes between 1-4MB.

If the average file copied to a drive is small – few KB rather than large GB ISOs – smaller block sizes ensure better utilization of the faster sequential throughput despite random seeks in between. There is also an interaction with operating system page size, buffers and kernel IO elevator that comes into play.

Benchmarking different block sizes

The optimal block size for a drive to achieve maximum write or read bandwidth can be determined empirically by benchmarking dd performance at different blocks. While recommendations exist for HDDs, SDDs and NVMe drives, real-world variations mean experimental validation is key.

A simple methodology would be to write/read a large file with dd at different power-of-2 block sizes starting from 4KB to 1MB. Tracking the average MB/s throughout the operation provides comparable bandwidth numbers to quantify impact of block. Doing this 3-5 times allows some basic statistics on optimal size.

For more rigorous benchmarking, tools like fio can simulate different IO loads across a sweep of block sizes and measure impact on bandwidth, IOPS and latency in a controlled fashion. However basic dd tests generally suffice for practical manual tuning.

Recommended sizes for common drives

Based on properties of common drive types like rotational disks, flash technology and modern interfaces – along with wins and tradeoffs visible during benchmarks – some rule-of-thumb block size ranges are:

HDD: Between 4KB to 128KB depending on RPM
SATA SDD: 512KB to 1MB
NVMe SSD: 1MB to 4MB+
Thumb drives, SD cards: 8KB-32KB

Enterprise devices could potentially use larger blocks but would likely max out bandwidth limits. We provide example case studies later for further insight into real-world optimized block size choice.

Use Case Examples

SSDs and NVMe drives

For a SATA SSD rated for sequential writes of 500 MB/s, benchmarking indicates peak transfer speeds are achieved between 512KB to 1MB block sizes in dd. This allows almost full saturation of bandwidth.

NVMe SSDs with sequential throughput reaching multiples GB/s correspondingly benefit from larger block sizes between 1-4MB based on tests. At the highest end, even 8-16MB blocks can sometimes be advantageous.

However optimal block size would also depend on actual IO workload. SSDs sacrificing some peak bandwidth for better random IO might choose 128KB or 256KB blocks instead. Light loads too favor smaller sizes. Still MB blocks works best for large sequential DD operations.

Hard disk drives

HDD rotation speed dictates some upper limits on usefulness of block size tuning in dd. 7200 RPM and 10,000 RPM drives achieve great performance at 64KB to 128KB blocks, also influenced by the underlying sector sizes.

Slower 5400 RPM variants typically maximize speeds at lower 32KB or so blocks beyond which returns diminish. Some parallelio with multiple threads and buffers can help push higher though. Enterprise SAS/SCSI HDDs could leverage up to 256KB blocks.

Thumb drives and SD cards

The interface protocol and access patterns of flash drives and removable media mean default block sizes underutilize their bandwidth. 8KB to 32KB blocks help them fully use their rated sequential performance. SD cards in particular benefit according to benchmarks.

However SD cards are also used for small random IO workloads where keeping blocks modest helps responsiveness. External SSD storage devices share similarities with their internal counterparts so favor appropriately higher blocks as their interface allows.

Tuning Block Size with dd

Syntax for setting block size in dd

The block size option in dd is specified in bytes using the bs flag followed by size as the first parameter:

dd if=/dev/zero of=/dev/sda bs=1048576

This writes zeros to device sda with a block of 1 megabyte or 1048576 bytes. Common sizes used range from 4096 (4KB) to several megabytes. The if and of arguments represent input and target output respectively.

Worked examples for common drives

Based on the preceding performance analysis and recommendations, here are some worked examples with tuned block size dd commands for cloning different drive types:

# 512GB SATA SSD 
dd if=/dev/sda of=/dev/sdb bs=1048576

# 2TB 7200 RPM HDD
dd if=/dev/sda of=/dev/sdb bs=131072 

# 1TB External HDD (USB 3.0)
dd if=/dev/sda of=/dev/sdb bs=65536

# 64GB NVMe Drive
dd if=/dev/sda of=/dev/nvme0n1 bs=4194304

# 32GB USB Flash Drive 
dd if=/dev/sda of=/dev/sdb bs=32768

Adjust if and of devices as needed. Block sizes follow rule-of-thumb ranges discussed earlier. Remember to validate and tweak for your devices as needed for extra performance.

Squeezing Every Bit of Performance

Tweaks beyond block size (buffers, threads etc.)

In addition to fine tuning block size, some further dd options can help extract even more performance such as:

Increasing i/o buffers with iflag and oflag
Parallel threads when cloning to multiple devices with count flag
Bypassing disk caches for true drive limits with odirect
Engineered high performance modes in status flag

Bumping buffer size can allow handling larger aggregates of even bigger block writes. Parallel dd passes divides workload for saturating bandwidth. Bypassing page cache avoids OS caching benefitting benchmarks.

Status flags like conv=fdatasync or conv=notrunc tweak low level operational details. Integrating these options along with fine-tuned block often cumulatively boosts dd performance beyond vanilla invocations in certain scenarios.

When sequential beats random access

A key principle that emerges from block size optimization in dd is how tuned larger sequential throughput universally exceeds small sized scattered random access on all modern drives.

Fundamentally this is visible in the order of magnitude difference between maximum sequential read/write limits and maximum random IOPS limits published in device specifications.

Real world performance matches this Spread. Exceptionally high terrabyte bandwidth versus modest tens of thousands of IOPS at best. DD’s block size tuning ensures our large copy operations achieve speeds closer to the former.

Even SSDs known for high random IO throughput gravely lag their sequential rates. And rule of thumb principles like “align blocks to erase blocks” recognize this now universal drive behavior pattern that dd block size optimization leverages.