ZFS vs. mdadm RAID — Why choose ZFS instead of RAID? ZFS pool vs. hardware RAID

ZFS and mdadm RAID both provide data redundancy, but ZFS offers distinct advantages that make it a preferable choice for many. Unlike mdadm, ZFS combines filesystem and volume management, simplifying data handling. It provides superior data integrity with its end-to-end checksumming and self-healing capabilities. ZFS also simplifies administration with features like snapshots and clones that are more efficient than traditional mdadm RAID arrays. For those prioritizing robust data protection and streamlined management, ZFS is often the better option.

Why Choose ZFS Instead of RAID — Core Technical Reasons

Checksums & Copy-on-Write

• Checksums for Data Integrity: At the heart of ZFS's data integrity promise is its extensive use of checksums. Every block of data written to a ZFS pool is first hashed using a strong checksum algorithm (such as Fletcher or SHA-256). When data is read back, ZFS recalculates the checksum and compares it to the stored value. If there's a mismatch, it indicates data corruption, which ZFS can often repair using one of its redundancy features like mirroring or RAID-Z. Traditional RAID solutions typically focus on disk-level redundancy without addressing block-level data integrity. This means silent data corruption, often termed "bit rot," can go undetected until it leads to significant data loss or errors.
• Copy-on-Write Mechanism: ZFS's copy-on-write (CoW) methodology further bolsters its integrity paradigm. Instead of overwriting existing data, ZFS writes changes to a new block, updating metadata pointers only once the write is successful. This approach not only protects data during unforeseen outages (e.g., power failures) but also eliminates traditional filesystem inconsistencies. CoW ensures that data and metadata consistently reflect a coherent set, rendering tools like fsck (filesystem check) unnecessary for ZFS.

Snapshots, Clones, Send/Receive

• Snapshots: ZFS's snapshot capability allows capturing the entire state of the filesystem at an exact moment in a near-instantaneous operation. Since snapshots are read-only and utilize CoW, they share unchanged data blocks with the original dataset. Therefore, they consume little to no additional space at creation, allowing administrators to retain numerous snapshots without significant storage penalties. The recovery from a snapshot in case of accidental modifications or deletions is as simple as restoring to that point, rendering them invaluable for rapid rollback and protection against data loss.
• Clones: Building on snapshots, ZFS allows the creation of writable clones. Clones are derived from a snapshot and, like the snapshot, initially share data with the original dataset. This feature makes them remarkably efficient for testing environments and developmental work where a replica of a production system is required for experimentation without affecting the live environment.
• Send/Receive Operations: The ZFS send/receive functionality streamlines synchronized data replication across ZFS systems. It facilitates incremental and differential updates between datasets, efficiently transferring only changed data blocks rather than entire files or datasets. This makes ZFS highly suited for backup strategies and disaster recovery solutions where bandwidth and time are critical factors.

Integrated Volume + Filesystem

• Unified Storage Management: ZFS unites file system management with volume management, eliminating the traditional, often complex, layering of volume managers (e.g., LVM) on top of RAID controllers. Instead of configuring static partitions, ZFS pools allow dynamic allocation across virtual devices (vdevs) and datasets. This encourages simplified storage provisioning where space is allocated as needed among datasets, avoiding the inefficiencies and constraints of pre-partitioned storage environments.
• Virtual Devices (vdevs) and Datasets: In ZFS, a pool is created using vdevs, which can be composed of individual disks or RAID configurations (RAID-Z). This flexibility allows ZFS to balance performance, capacity, and redundancy tailored to specific needs. Within a pool, datasets can be created that inherit properties such as compression, deduplication, and quota settings, offering granular control that isn’t generally available with traditional filesystems layered over RAID.
• Scalability and Flexibility: This integrated approach simplifies adding storage to existing pools. Instead of modifying or dismantling existing RAID arrays, administrators can add new storage devices (or vdevs) to a ZFS pool, instantly increasing capacity and distributing data across the expanded pool. This facilitates scalable growth and enhances the long-term utility and manageability of storage systems built on ZFS.

Performance & Tuning — What Affects Throughput

ZFS Tuning Factors

• ashift: This parameter dictates how ZFS aligns data blocks with physical disk sectors. It's crucial to set ashift correctly according to the underlying drive's sector size (usually 4K for modern disks) to avoid performance degradation due to misaligned I/O operations.
• recordsize: Determines the size of a block in a ZFS filesystem. Adjusting this can optimize performance for different workloads; for example, smaller record sizes may benefit databases, while larger ones might be better for sequential read/write operations typical in media storage.
• ARC Size: The Adaptive Replacement Cache (ARC) is ZFS's in-memory cache. Its size directly impacts read performance. Allocating more RAM to ARC can enhance read speeds and overall system responsiveness, but must be balanced with the system's memory needs to avoid resource contention.
• SLOG (ZIL): The Separate Intent Log (SLOG), often referred to as the ZFS Intent Log (ZIL) when cached on an SSD, can improve synchronous write performance. Although not always necessary, deploying SLOG devices can offer significant speed improvements, particularly in environments that perform frequent sync writes.
• L2ARC: This is an extension of the ARC, residing on SSDs, that helps increase read performance by keeping frequently accessed data closer to the CPU. Adding L2ARC can significantly accelerate access times for systems with predictable, repetitive read patterns.

Optimal performance tuning of ZFS involves considering these factors as per the OpenZFS hardware guidance, allowing for tailored adjustments to meet specific workload demands.

mdadm / Hardware RAID Tuning Factors

• Stripe Size: This refers to the amount of data written to each disk in a RAID array before moving to the next disk. Matching stripe size to typical file sizes or I/O patterns can maximize performance. Smaller stripe sizes might benefit lots of small writes, whereas larger stripes improve performance for large sequential data transfers.
• Controller Cache: The cache on a RAID controller can dramatically enhance performance by buffering writes and reordering them for optimal disk access patterns. Setting the cache correctly influences RAID performance, especially in write-heavy workloads.
• Write-back vs Write-through: In write-back mode, data is cached by the controller before being written to disks, improving write speeds but requiring battery-backed cache to prevent data loss in power failures. Write-through, on the other hand, writes directly to disk, providing secure data writes at the potential cost of reduced performance.
• Queue Depth and Driver Behavior: Queue depth determines how many I/O operations can be queued at the controller at once. Finding the right balance for queue depth and ensuring drivers are optimized for the specific hardware ensure that the system can handle the expected I/O load efficiently without inducing latency.

perational Considerations — Management, Monitoring & Backups

Monitoring & Scrubs

• ZFS Scrubs: ZFS incorporates a powerful and automated data integrity check called "scrubbing." A scrub systematically checks all the data and metadata in a storage pool by verifying checksums against the actual data. If any discrepancies are detected, ZFS attempts to automatically repair them using redundancy if available. This process helps to proactively detect and fix issues before they lead to data loss. Scheduling regular scrubs is a critical practice for maintaining data integrity in a ZFS environment and should be implemented as part of the routine maintenance strategy.
• mdadm Monitoring: In contrast, mdadm does not inherently perform data integrity checks at the filesystem level. Instead, it focuses on managing RAID arrays, and any filesystem-level integrity checks, such as using fsck, must be conducted separately. Moreover, regular health checks and status monitoring of the RAID array need to be manually integrated into maintenance routines to ensure reliable operation and to spot potential issues promptly.

Backups & Disaster Recovery

• ZFS Send/Receive: One of ZFS's most valuable features in the context of data protection and disaster recovery is its ability to perform send/receive operations. This feature allows for efficient replication of data from one location to another, facilitating robust disaster recovery solutions and enabling administrators to create mirrored copies of datasets across different systems quickly. While ZFS send/receive is highly efficient and can significantly enhance data resilience, it should not be misconstrued as a replacement for traditional backup solutions.
• Maintaining Off-Site Backups: Despite ZFS's capabilities, maintaining off-site backups is an essential component of a comprehensive data protection strategy. Off-site backups guard against data loss scenarios that affect the primary and secondary sites, such as catastrophic failures, natural disasters, or ransomware attacks. Thus, implementing a comprehensive backup strategy that includes traditional backup methods, possibly augmented by ZFS's send/receive, provides the best coverage against data loss and ensures quick recovery in the face of data integrity threats.

Migration & Compatibility — Moving from mdadm / Hardware RAID to ZFS

Migration Options

• Send/Receive: If you already have your data on a ZFS system and are looking to migrate to another ZFS setup, utilizing the ZFS send/receive feature can be an effective method. This tool allows for efficient transfer of datasets, including snapshots, from one ZFS system to another, preserving all filesystem properties and history.
• Rsync from Mounted Filesystems: For systems that aren't initially on ZFS, using rsync to copy data from mounted mdadm or hardware RAID filesystems to a new ZFS pool is a practical approach. This method provides fine-grained control over the migration process, allowing you to ensure consistency and integrity of the files being transferred.
• Temporary Storage Migration: In scenarios where direct migration isn't feasible due to hardware constraints, using temporary storage as an intermediary can facilitate the migration. This involves copying data from the existing system to a temporary storage solution and then transferring it onto the new ZFS system. While not the most efficient, it's sometimes necessary for complex migrations where space or hardware is limited.
• Avoid In-Place Conversion: It is generally not advisable to attempt an in-place conversion of controller-managed RAID setups directly to ZFS without first creating full disk images. Such operations are inherently risky and could lead to data corruption or loss. Instead, perform a fresh setup of the ZFS pool and migrate data through one of the safe methods mentioned above.

Testing & Verification

• Benchmark Workloads with fio: Before performing a full cutover to ZFS, it's crucial to benchmark expected workloads using tools like fio. This will help you assess if ZFS deployment meets performance expectations and allows you to tweak system configurations as necessary.
• Validate ashift: Ensure that the ashift value, which aligns ZFS block size with the physical disk sector size (often 4K in contemporary drives), is correctly set. This can prevent performance degradation due to misaligned I/O operations.
• Test Scrubs and Resilvers: Perform scrubs and resilver tests on non-production data to confirm that ZFS is correctly configured to maintain data integrity and redundancy. These tests will help identify potential issues in the redundancy rebuild process and verify that the system can automatically correct data errors.

Comparison table — features, pros & cons (summary)

Quick Cheatsheet & Commands

ZFS Diagnostics

• zpool status: This command provides a summary of the health and status of all pools on the system. It details each pool's configuration and identifies any errors or issues with the drives.
• zpool scrub: Initiates a scrub of the specified pool. A scrub thoroughly examines all data blocks within the pool, verifying checksums and attempting to repair any inconsistencies using redundancy.
• zdb -C: Displays configuration information for all known pools. This tool is especially useful when dealing with pool copies or images, providing insight into the internal structure and metadata of ZFS pools. It should be used carefully, typically on non-live data to avoid impacting active operations.

mdadm Diagnostics

• mdadm --detail /dev/mdX: Provides detailed information about a specific RAID device. This includes component disks, RAID level, state, and any ongoing recovery operations. Replace /dev/mdX with your specific RAID array identifier.
• cat /proc/mdstat: Shows the status of all active RAID arrays on the system. This includes brief information about the state and health of each RAID array, which is useful for quick checks and ensuring everything is functioning as expected.